Imaro
Legend
Actually, the Complex Skill Checks option in the 3.5 UA is very, very similar to the Skill Challenge mechanic in 4e. So much so that, when the inevitable 5e comes along, the 4e retro-clone (should one exist) should be able to use the OGL UA material without any problems whatsoever.
It'll be the classes, powers, and adjectivenoun creatures that are difficult to retrofit without violating IP.
RC
Yeah, I was aware of the UA complex skill checks, and agree they are very similar to skill challenges... however I've brought stuff up before from UA to make points and there's always cries of "That's not core rules though... it's optional". Just didn't feel like going down that particular road again. In the end though, I just don't think 4e's skill challenges really do anything for the game that can't be done with normal skills.