Bootloader freezes on login screen when booting into Lion from Boot Camp
This suggestion seems to helps a lot with all sorts of things, so I figured I'd suggest it here just in case.

Go to your Disk Utility app and press the following buttons in the numbered order. I would recommend only doing 1, 2, and 3, but if this doesn't help with the problem, you can also try to run 1(b) from Recovery Mode (or while running your computer off an external hard 
drive) because it won't work while you're logged on.



Hope this helps :)