Some jerk at the Apple Store at the Eaton Centre in Toronto told me that using non-Apple RAM does void the warranty. He was a very cocky sales rep, and was really trying to hard-sell me the MacBook. When I disagreed with him and asked him to show me that in the warranty, he insisted he was right and just pointed at a MacBook and told me to look it up on Apple.ca, and then walked away. That's a first at the Apple Store for me.
He also told me that you need a Superdrive to read DVDs, and that a 2.2 Ghz processor is a very noticeable speed difference from a 2.0 while using the MacBook for common tasks (iLife, Office, Safari, etc).
I walked straight over to another rep, asked the same questions, and got the right answers (doesn't void the warranty unless you damage during install, Combo drive reads DVDs, not a very noticeable speed difference for common task usage). I, of course, pointed out the rogue sales rep and suggested that she educate him on the products he is trying to sell.
/still shaking my head.