Whenever I go from fullscreen to windowed mode my screen becomes black, but I can continue doing everything as normal (just can't see anything). Going back to fullscreen displays whatever the application should be displaying.
I'm using Windows 7 64bit and I have an ATI HD4850. My friends with NVIDIA cards do not run into this problem. Bug is kind of similar to the white screen Mac issue in http://lwjgl.org/forum/index.php/topic,3987.0.html