I also am experiencing graphic artifacts but only while using Safari. These artifacts appear when I am going into Top Sites and only lasts for a fraction of a second. The actual appearance of the artifacts changes. Sometimes it appears as a mostly gray screen with streaks in it. And sometimes it appears as a distorted version of the screen I have just left.
It happens all the time I am going into Top Sites, regardless whether I have just started my day i.e. the computer is cool, or 10 hours later when the computer is hot.
I have a 27 inch iMac from early 2010.
System 10.6.8
Model Name: iMac
Model Identifier: iMac11,1
Processor Name: Intel Core i5
Processor Speed: 2.66 GHz
Number Of Processors: 1
Total Number Of Cores: 4
L2 Cache (per core): 256 KB
L3 Cache: 8 MB
Memory: 8 GB
Processor Interconnect Speed: 4.8 GT/s
Boot ROM Version: IM111.0034.B02
SMC Version (system): 1.54f36
Hopefully our different stories will help Apple comprehend and fix the problem.
Joe