Received Leopard today (UTD). Thought I'd give it a shot (if it locks up all the time, I can actually prove the freezing). Instead of updating, I chose a clean install.
Anyway, during the installation, small artifacts were showing at the menubar, but it didn't freeze. Installation ended after about half an hour, the computer rebooted, showed the funky "welcome" movie (God, I hate that song). After the movie, a window popped up to pick the country, but couldn't actually pick anything since the computer froze. I rebooted, the movie played again (God really must hate me), the movie ended and I saw a faded glimpse of the select-country-window, but the computer had frozen again. I rebooted yet again, the movie played yet again (this time I muted my speakers), window pops up, and by Zeus! It worked.
Anyway, onwards to the desktop, artifatcs were showing here and there, but nothing too major. I installed all the updates (yes, I like russian roulette) but the computer froze when I hovered over the dock right after boot. So I did another reboot, followed by a PRAM reset, got back in Leopard and no artifacts showing!
I played around a bit with Front Row, Fired up Arabesque screensaver for about 2 minutes. Spammed the dock with my cursor (and coverflow as well), Played WoW a bit in fullscreen with all settings max'd (excluding AA), no artifacts or freezing. Now I'm typing here while throwing my mouse around over the dock, still no artifacts nor freezing.
I expected worse tbh, I was preparing myself for a whole night of frustration with a freeze every 5 minutes.
Now, I do know that I still have a freezer, though, and I'll keep waiting for a bit to see if Apple comes up with something.