Did you just install OSX Lion and XCode 4.1? Does it crash on you as soon as you try to launch it?

Your problem may be solved by simply re-installing iTunes. Yes, I know what you're thinking... "What the heck does one have to do with the other?" I have no idea. But this appears to resolve many people's problems. Just go to http://www.apple.com/iTunes and download it and install.

See this discussion for more details: https://discussions.apple.com/thread/3188708?start=15&tstart=0