Some guys on a very large message forum that I post on claim they made this video and found this glitch. All I gotta say is, wow! You can jump pretty much anywhere you want in the game, with a friend handy of course!
http://www.nhgz.com/hosted/oryan/videos ... LowRes.wmv
Right Click -> Save As...
I don't have Halo 2, but I'll probably end up getting it 'cause a large group of my friends likes playing the game together.