I've had a problem twice now with my new system. It's an AMD Athlon X2 4200+ with an Asus M2NPV-VM motherboard running OpenSuse 10.1 Linux. What happens from my point of view is that the display locks up, that is the image is frozen - doesn't move, and the mouse pointer won't move. It doesn't respond to keyboard input either. Once there was sound playing (from two sources) and it just looped one second of audio endlessly. I tried pinging it from another machine on my LAN and there was no response (after I rebooted I could ping it, so I know it's pingable). The first time I was surprised that some little game I was playing was able to lock up Linux. The second time the symptoms were identical but I was running mostly different applications.
I leave this computer on all the time so if this were happening often I'd expect to come home and find it locked up. That hasn't been the case - when I'm away from home I occasionally ssh in and have no issues whatsoever.