I also have EXACTLY THE SAME PROBLEM on my MBP i7 15" here's a picture:
http://www.flickr.com/photos/37518869@N08/5211145142/sizes/l/in/photostream/
I get these distorted graphics everyday while using safari, specially happens on Top Sites, on Gmail, Youtube ... I took my MBP i7 to Apple Care Center, they said they run all possible tests and didn't found any problem at all.
So I got my computer back from Apple Service Center on Nov 6 (yesterday) and it is still happening... After reading all your posts on this thread I know that is totally worthless to replace the logic board or anything since this exact issue is happening to all of us.
So what we can do besides of waiting for Apple to take action ?