I'm having lots of problems with this too. It only happens when I'm running Safari 5.0.1 (6533.17.8) on 10.6.4 (10F569) on a 15" i7.
When temperatures reach ~130ºF while using the Intel graphics card and Safari, I get a full screen of artifacts. It usually begins when I load a page in Safari (with or without flash content) and then scroll. Then a rectangular piece of the middle of the page will stay in place while the rest scrolls. Then, the whole screen is artifacted. Taking a screenshot using Cmnd-Shift-3 yields a solid black image that's not my screen resolution.
If I switch to the NVIDIA graphics, everything is back to normal. But when I switch back to Intel, even after I quit Safari, my screen is still covered in artifacts and little pieces of the website I was browsing, and pieces of sites I had visited even before there were any issues.
The funny thing is that all the artifacts are below the OS X menu bar (meaning I can see the artifacts through the translucency, but the menu bar itself is fine and operates normally) which makes me think it is a software problem. Also, sometimes my wallpaper, which is the Snow Leopard default, reverts to solid blue, but System Preferences shows that it's still the default wallpaper selected.
I remember that older MacBooks had a problem with artifacts if secure virtual memory was turned on (System Preferences -> Security -> General). I haven't had a chance to try turning it off, but has anyone else had any luck?