I have just installed Natty on an Intel Celeron single core CPU with 1 GB RAM without a 3D graphic card and Natty is running on Classic Desktop instead of Unity.
I have to force turn off and restart the computer system to recover.
Maximizing, minimizing and restoring do not freeze the system. Only when I tried to click and drag the border to resize my computer system freeze with the window having a orangy tint.
Otherwise Natty run along nicely for now.
Best Answer
I see you commented on your own question. Just wanted to give you some options:
You can install 2d-unity as a work-around:
Then you choose it from the login window as "2d Unity", I think. This will look nearly the same as the default ubuntu desktop, but without 3d acceleration.
Also, you may want to file a defect in launchpad if you can reliably reproduce the crash. If you really feel like narrowing it down, you could open up "CompizConfig Settings Manager", and try turning of some features like "animations" or "fading windows" and see if one of those features is causing the crash.