After the third time my motherboard died in three months, I was madder then hell and Apple agreed to replace it. I had the 800/combo and ended up getting a 900/combo as a replacement.
They might try to tell you that they can't replace it outright since the newest models are better then what you have. This is crap, don't listen to it. Keep the pressure on them, tell them how you can't be without your machine, etc, and they'll relent. I had to do that with mine, given that I use it constantly for school and as my main machine more or less. Being without it for a week was not an option and the two times it went it, I was in a serious bind at school. Just be nice but firm when you call tech support.
-Jeff