I replace mine when I first got my MBP. In fact, the 500g drive fail to work. I can apple care and ask them why. They said that I have void the warranty at first. But I ask them, if by doing so void the warranty, why do they put the instruction in the manual?
Anyhow, after a few chat, they agree that it does not void the warranty. They send me a replacement MBP because I told them it has problem with HD more than 250GB. The new replacement I got work perfectly.
I don't know what happen to the first unit I got, but it only works with 250GB or below. it is very strange.
So, I said check your manual for instruction and go for it. It doesn't has warranty seal for memory and hard drive.
if this answer is helpful to you, I appreciate it if you can give me some score. thanks!