Some users are experiencing graphical problems with their iMacs, and it appears that the anomaly related to OS X Lion. A quick survey at Apple Support Communities (formerly Apple Discussions) shows us that at least two topics (1, 2) have already been opened for deal with it.
As we know, every transition requires special care, and Snow Leopard to Lion would be no different. Some people (I include myself in this group) did not have any problems with migration, but that did not prevent others from facing several headaches (1, 2, 3, 4, 5).
David Martin, editor of Cult of Mac, is part of this team and posted some photos showing the bug in ao:
The problem he faces is very similar to what users of 27-inch iMacs (configured with 3.4GHz Intel Core i7 chip and ATI Radeon HD 6970 video card with 1GB of GDDR5 memory) are experiencing intermittent, and always disappear for a time after restarting the machine.
Martin contacted AppleCare but they were unaware of the bug and, obviously, no fix is ??available. On the other hand, after reporting the problem to Apple, he received a different response from the engineering team: they are, yes, aware of the bug, which may not even be specific to iMacs. However, it does not appear to be something related to hardware, which allows for a correction via Software Update (Software Update).
Although Apple engineers are aware of the anomaly, no information about when a correction arrives has been said. But if you are facing the same problem, at least you already know that it will be solved perhaps with OS X 10.7.2, which is about to come out of the oven.