I'm having this same problem with Kubuntu 11.10. I have the same graphics card in my Thinkpad x100e (ATI Radeon HD 3200) and AMD Athlon Neo X2 Dual-core processors.
Just 2 days ago, I did a fresh install and had the same random screen blanks/system freezes. I tried using the ATI proprietary drivers, and that just makes the random system freezes happen at different times.
I've turned off desktop compositing, and set my power devil to not blank or dim the screen in any mode.
I can't discern a pattern, and it's a real spirit-breaker. This thing's always been a bit finicky, but not since 10.04 have I had such issues with it.