I have the same problem like you.
I'm running Backtrack 5 KDE 64bit. It works fine in live DVD mode (support 1920x1080) but when i installed on HDD, it goes only to 800x600.
I've tried to some ways to change screen resolution but still not working.
I use laptop AlienWare M15x with NVIDIA GeForce GTX 460m.
Any ideas will be appreciated.