Hi everyone,
My first post in this great forum.
I had the same symptom: white screen and stuck mouse while the installation of osx Mountain Lion. I noticed the flash drive I was installling from and the external HD I was installing to were both blinking, so I let it continue installing.
Guys, as crazy as it sounds, sometimes for some of you the simplest solution would be to 'press a key' Yes, that's it. The screen will come back and you'll see the installation is in progress. Just reporting as it might happen to others.
Cheers