Opening Thoughts
NB: This is a PvE build first and foremost. If you want PvP, either don't roll an inv tank (they aren't all that good at it) or, better yet, go ask someone who doesn't think PvP is a badly thought-out, pointless, bolt-on, ego-stroking, lamer-justifying, griefing device.
Invulnerability's seen some major nerfs lately. Between I5 and I6, we've seen our resistances having been gutted, and we are a shadow of our former...
Wait, lets start again. Invulnerability has met several challenges in the recent months. However, as a powerset and as an AT, the Inv Tank still stands as an example of unbreakability. Oh sure, you now have to do a little more work than you used to, but it's still fun.
Slaps and Bennies (AKA. Whats in it for me? and what sucks?)
Bennies
Survivability: Even today, you're eminently survivable. Decent S/L resists, combined with lesser resist to other types. Not exactly the golden age,but perfectly adequate.
Hitpoints: Dull Pain is Your New God. Accept it. What was merely nice to have before now becomes vital. With Dull Pain up, you can get something in the region of 3000 hit points at level 50. Not shabby.
Damage: Broadsword scrappers have massive damage at high level. I know this. Energy Melee however, at 35 and 38 has two attacks that eclipse broadsword, the twin holy grails of the EM set. Energy Transfer and Total Focus. ET can practically one-shot minions, and TF isn't far behind. ET uses comparatively low end, instead taking a certain amount of your (3000odd) hitpoints, which tends to surprise the empath that's watching your health bar (sorry Hikari

), and is a liveable side-effect of taking out the foe that quickly. TF does slightly less damage, and costs more, but is practically a one-shot mez against anything short of a boss, and even has a great chance of stunning bosses too. I've even stunned AV's with it before for a few seconds.
Slaps
Psi: Against Psi damage, you're weaker than a wet fish, and its prevelant in the 40+ game. Your only defence here is your huge hitpoints and the other thing you get in the late game, your huge single-target damage.
Endurance Use: It's a hog, never doubt that for a moment. Get stamina. You will need it. Slot those toggles with end reduction. Slot the attacks in the late game, and you'll be a happy munchkin.
The Build
Disclaimer: The travel power was as much concept as utility, and you can probably alter it to suit without pain (and make the build maybe slightly more efficient)
01) --> Resist Physical Damage==> DmgRes(1) DmgRes(17) DmgRes(23)
01) --> Barrage==> Acc(1) Dmg(13) Dmg(33) Dmg(40) Rechg(43)
02) --> Temp Invulnerability==> EndRdx(2) DmgRes(3) DmgRes(3) DmgRes(5)
04) --> Energy Punch==> Acc(4) Dmg(5) Dmg(34) Dmg(37) Rechg(43) EndRdx(48)
06) --> Dull Pain==> Heal(6) Heal(7) Heal(7) Rechg(13) Rechg(27) Rechg(29)
08) --> Unyielding==> EndRdx(8) EndRdx(9) DmgRes(9) DmgRes(11) DmgRes(11)
10) --> Taunt==> Rechg(10)
12) --> Recall Friend==> Rechg(12)
14) --> Teleport==> EndRdx(14) EndRdx(15) Range(15) Range(43)
16) --> Bone Smasher==> Acc(16) Dmg(17) Dmg(34) Dmg(37) Rechg(42) EndRdx(48)
18) --> Invincibility==> EndRdx(18) DefBuf(19) DefBuf(19) DefBuf(21) TH_Buf(21) TH_Buf(23)
20) --> Hurdle==> Jump(20)
22) --> Health==> Heal(22)
24) --> Stamina==> EndMod(24) EndMod(25) EndMod(25)
26) --> Whirling Hands==> Acc(26) Dmg(27) Dmg(34) Dmg(37) Rechg(40) EndRdx(48)
28) --> Resist Energies==> DmgRes(28) DmgRes(29) DmgRes(31)
30) --> Resist Elements==> DmgRes(30) DmgRes(31) DmgRes(31)
32) --> Unstoppable==> Rechg(32) Rechg(33) Rechg(33)
35) --> Energy Transfer==> Acc(35) Dmg(36) Dmg(36) Dmg(36) Rechg(46) EndRdx(46)
38) --> Total Focus==> Acc(38) Dmg(39) Dmg(39) Dmg(39) Rechg(40) EndRdx(46)
41) --> Conserve Power==> Rechg(41) Rechg(42) Rechg(42)
44) --> Focused Accuracy==> EndRdx(44) EndRdx(45) TH_Buf(45) TH_Buf(45)
47) --> Hover==> Fly(47)
49) --> Energy Torrent==> Acc(49) Dmg(50) Dmg(50) Dmg(50)
Options, thoughts and customisations.
Teleport: I like teleport. Many don't. The hover eventually comes necessary to protect against lag, but you can drop that if you need to. Or you could go SJ or fly with equal utility. I use teleport in combat for sneaking up on sappers, and getting out of quicksand/caltrops, if you get annoyed with Knives of Artemis or Earth Thorn Casters.
Unstoppable: Divided Opinions. 50% hate it, 50% love it. Massive resist boost, tasty endurance regen and health regen boost and extra mez protection for 3 minutes. After 3 minutes, however, you crash to minimal health and end, which usually ends up dropping all your toggles and leaving you vulnerable, if it happens. This tends to
really surprise empaths who haven't seen it before (again, sorry Hikari

) This can be mitigated with planning, and sometimes, if the drop times right, you don't lose all your end or toggles (not sure how to make it consistent, though) What is sure is that unstoppable is an "oh poot" power
par excellence, and, anyone who has seen an unstoppable inv tank running away, with a mob of about 20 or 30 bad guys all throwing huge attacks at it, with the rest of the team wiped on the floor, is unlikely to forget it. When you hit that button, you have three minutes of virtual godhood. Remember the drop, factor it in, and unstoppable will love you and hug you call you george. If you're still taking huge damage through it, either you're facing an AV, something throwing psi at you, or you're in so much trouble that nothing's going to save you.
Invulnerability: Not as grand as it once was, but still, a defence boost per enemy in melee is not to be sneezed at, increases your survival time a fair whack, and is generally a good thing. It's an open debate as to whether slotting with To Hit buffs does enough good. I like it, so I do it. You don't have to.
Epic: I chose energy mastery for conserve power and focussed accuracy. feel free to choose another if you think the hold will do you more good than the end help or acuracy/perception boost.
Taunt: The most hotly debated point. I put 1 recharge into it, and called it golden, between that and punchvoke, I can handle most things, and it recharges fast enough for my play-style. YMMV, so see what works for you.
This build is balanced between decent soloability (especially in the late game) and decent team play. You could tip the balance one way or the other if you wanted, with things like health pool. I don't recommend fighting pool, if only because I don't believe the extra boost from tough/weave is enough to justofy the extra power/enhancement slots and end use (and the fact that every attack you get, including the first, are still better than boxing or kick, so there's a wasted power right there.) I suppose you could get concealment pool, but you're a tank, and with your autotaunt aura up, it kind of defeats the object somewhat. I don't believe it useful enugh to take. People used to as the pre-req's for phase-shift, but with the nerfing of that, the concealment pool became an expensive liability for a tank.
Stun and Build Up. If you were a scrapper, I;d say get build up. But you aren't, and there are far more useful powers to get. Same with Stun, in PvE there are far more useful powers to get and you don't have the power slots to justify it. Again, YMMV.
Final Words
To summarise. A fully solo-capable team-tank, with decent survivability, and a whole slew of decent damage powers. Treated right, and played with a little intelligence as to what you do, how you do it, and where you do it (ie. not just mashing buttons, mmkay?) and you'll get years of happy play from your inv tank. That is, until they decide they're overpowered again.