I have the 27" 3.06ghz dual core
It's my second, as the first one had the flickering screen. I received some of the best customer service I've ever encountered when contacting Apple for a replacement. They admitted fault immediately, told me there was a problem with the hardware and I took it back to the store to be given a new replacement within five minutes of arriving, complete with gushing apology. Better still I only brought back the machine, not the accessories, so I gained an extra magic mouse and wireless keyboard in the process.
This one has been absolutely perfect since I got it a month ago and I love it to pieces. I wonder just how bad this problem really is - after all, those without a problem don't shout about it.