I believe it works by creating a blessed folder (/efi) which is set to be accessed before the normal system files.
As far as i know it doesn't void the warranty, i have been using it for ages now, no problems what so ever. Very easy to remove if you no longer want it.
It shouldn't. The only trouble I had with rEFIt was that I couldn't restore the hard drive to just the OS X partition, even after going back to the normal Macintosh bootloader.🙁