After from talking to applecare reps on the subject, my understanding is that unless you get an apple authorized service provider to put in a hard drive that is offically supported by apple for your machine, you're voiding applecare.
Yes, you could quote various state and federal laws, and try to make it apple prove that the hd swap caused your problem.
That being said, I want a warranty, not a potential lawsuit. If you need a bigger hd, just get an external one and hook it up using firewire.
And by the time mine's out of warranty in 2011, they'll probably have SSD's (STD's?) for the same price of today's hd's.