I want to upgrade my harddrive to a SSD. Does apple offer this service? I was going to do it myself, since its easy. Does this void my warranty?
No, it will not void your warranty. If you refer to the top of these instructions, they clearly state only a deviation of the steps outlined may void your warranty:
Typically, anything that is printed in the owner's manual is covered under warranty. RAM and changing the hard drive are both topics that are carefully covered, so legally speaking, that means they are user serviceable. Anything user serviceable is covered under warranty (following the above proviso of course).
Unless it explicitly states in the manual that such modifications must be performed by a qualified technician, you will not be in breach of your warranty.