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:
Follow the instructions in this document carefully. Failure to follow these instructions could damage your equipment and void its warranty. Online instructions are available at http://www.apple.com/support/diy/
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.
The link above states the following:
"We address this topic directly with customers via our support portals and are happy to inform you here of the same fact: upgrading your Mac does not void its warranty. This consumer protection is owed to the little known Magnuson-Moss Warranty Act of 1975. Put simply, the act states that a company can’t require you to upgrade with only its own branded parts to retain the warranty. This important act protects your rights as a consumer and allows you to install OWC upgrades with peace of mind confidence."