I had this same question. I just bought 8GB of RAM and attempted to install it myself. However, I had trouble with one of the screws starting to strip and so I stopped. When I called one of my local Apple stores, I was told emphatically that had I opened the laptop and installed the RAM myself I would have voided my warranty. This was told to me after consulting one of their on-site geniuses! Since I have had this laptop for less than a month and I did purchase the Apple Care extended warranty I would have been so upset!
The guy at the store told me the only way to upgrade without voiding the warranty is to 1) purchase the RAM from them ($200 vs the $40 I paid) and have them install it or 2) go to an Apple-certified repair shop. They would not install RAM I purchased myself.
Now I did read the manual and do agree that it seems to state that damage resulting from an inproper RAM (or HD) install would not be covered by the warranty. But considering the response I received from the folks at my local store, it makes me wonder if attempting to make a warranty claim in the future would be met with resistance from Apple if they were to determine that an "unauthorised" RAM/HD install had been made on the machine.
Does anyone have any live experience with having done an upgrade themselves and still having their warranty honored...or not honored, as the case may be?