Xcode 4.1 GM Not Installing (must quit iTunes) on Lion?
Looks like you've solved the install problem. It's not a solution per se, but setting Xcode to run in 32-bit mode fixed the crashing problem for me.
It's too early to say what the trade-off is (if any), but whatever it is, it's bound to be better than not being able to use it at all.
On a side note: please accept Phil's answer since he gave the correct solution to your original problem. C: