Shemeska
Adventurer
I'm surprised Planescape isn't leading this poll by a country mile, though; It's definitely what the rest of my group would have voted for.
There's a difference between just having a planar campaign setting and doing the Planescape setting. I don't think it's possible for 4e to do the latter unless they're willing to create a setting that breaks from core 4e tropes and doesn't force 4e core flavor and concepts into the setting.
Planescape tied to 4e/PoL would lack the full alignment spread which was one of the major defining focuses it had, it would lack half of the iconic planar races, there would be no Blood War which was the setting's major driving conflict, and there would be no Great Wheel cosmology, etc etc etc. It would end up as the 4e planes with some Planescape elements thrown in, but it wouldn't be the same setting unless WotC was willing to change 4e elements to accomodate the setting rather than change the setting in massive ways to accomodate 4e.
That's why I didn't vote for Planescape on the poll. I voted for a unique setting. Be creative, show their material for what it is and frankly that would be served by making something new rather than remolding older settings which came out of a very different game than 4e.