Some users of early 2009 iMacs who have upgraded to OS X Snow Leopard or higher are still reporting issues with a kernel issue that seems to be due to the Nvidia GeForce GT 130 graphics card that came with the machine, with nary a response from Apple proper. There’s a thread on Apple’s support discussion pages that is now around a year old that mentions the problem. According to the posters there, there was a faulty kernel extension released in one of the later Snow Leopard updates that can cause graphics glitches and even kernel panics when there’s a heavy load on the video card, like when playing games. Apple has not yet responded in the official forums.
2009 iMac Users Still Complaining Of Kernel Panic, Unresolved Since Snow Leopard