Does installing rEFIt void my warranty?
No, rEFIt is software and therefore should not affect the hardware warranty.

However, if you're concerned about that and want to avoid trouble (there's always the odd thickheaded customer service rep, even at Apple), you can simply wipe your drive and do a clean OS X reinstall before you send your machine in for service (and restore from backup once you get it back). rEFIt does not modify your EFI or anything else, so there'll be no trace left of it if you do so.