Just discovered this The Hard Way, but if you run Boot Camp; make sure you get the Boot Camp 4.0 update on the Windows side before you turn on FileVault. If not; Windows will try to mount your Mac volume (which is now encrypted gibberish), fail miserably, and BSoD every time within 30s of logging in. If it's too late, booting into safe mode and renaming the HFS drivers (AppleMount.sys and AppleHFS.sys IIRC) in C:\Windows\System32\drivers will get your system out of limbo.