I've been following this discussion for about 7 months now... .ever since my 27" iMac (mid 2011) started freezing for no distinguishable reason, along with screen black outs and artifacts, or random system shut downs. The problem was so random and so varied it was impossible to find the source.
Now, I do love Apple products, but in my experience I try to avoid dealing with Apple geniuses because generally they're not much help. I'm painting broadly here, but there's nothing worse than "Are you sure the power cord was plugged in?". That might be helpful for my grandfather, and I can appreciate that on some level; but not when I have projects that are on hold because of this issue.
Instead of going to an Apple store, I took my computer to a local computer shop that has an Apple Certified Technician. It was much easier to build a relationship with this guy and prove to him in a few short conversations that I did indeed know what I was talking about and already went through a wide variety of troubleshooting tecniques. Despite me being unable to reproduce the problem in store, he was willing to take the system and perform some tests of his own. (Now, I should mention that I do have Apple Care and I'm sure that helps make things run smoother... but technically it was still under 12 months, so the normal warranty should have worked as well.) As a expected, his tests could not detect any problem, and the iMac ran just fine for the two days he had it.
Now, here's the catch - I'm sure this local store answers to Apple in some way, but not the same way the employees at an Apple store do. "Geniuses" will eventually toe the party line because they have to; I wouldn't expect them to easily admit to an expensive problem, especially if they can't reproduce it in store. But I do believe my local computer shop guy is more concerned about helping the customer. I told him he needed to do something to it because I wasn't going to take it back without trying something. The issues seemed related to GPU or logic board, so he agreed to replace both of them. BOTH! I was happy to hear it.
But the problems didn't end there. The logic board and GPU were replaced, but after a few days of normal use my newly repaired iMac completey died and wouldn't even turn on anymore. Finally! A problem I don't have to reproduce. I called Apple Care, and after checking to make sure my iMac was actually plugged in, he recommended I send it back in for another repair. So off it went. My local shop replaced the logic board and GPU again.
Alas, this did not seem to solve the problem. When I got my computer back it started freezing on me just like it did before. I called Apple Care again, and they were still convinced that it must just be a bad logic board. Really? Don't you think there must be something else killing the logic board? What are the chances? Nevertheless, it was replaced a third time.
When I got my iMac back, it ran pretty good for about two weeks, and then half of the LCD went dim. It works, but I can't exactly use it for colour grading video productions. I called Apple Care again, and said that three repairs was enough for me. I wanted a replacement unit. He asked me if I wanted a refurbished 2011 model, or a new 2012 model. I opted for the brand new 2012 model (wierd, right?). It should be coming soon, and this whole ridiculous ordeal will be over.
If you don't have Apple Care, get it while you still can! It will save a lot of stress. They spent $2200 on repairs for my iMac before finally agreeing to replace it with a new one. I spent $300 on Apple Care.
Sorry for the ramble. 🙂