Ugh, what were you smoking when you said yes? We do weddings at TriDuo and I refuse to work them unless absoultly needed.
Bodies and flashes already mentioned, fast lenses are a plus.
If you can, use the ambient light, candles and dim tungsten make for romanticesque shots. If you have the ability or can mooch from a friend, set up a couple of remote strobes above the dance floor for the mom/son daddy/daughter ect dances. Fill flash on camera if the light requires it.
Use the Lo/hi ISO/range setting on the 5D for groom and bride shots, shoot raw or raw+jpeg.
Don't forget the fun shots of the dads getting plastered, the bestman ribbing the groom and such. Get artsy shots of the guestbook, cake, invitations.
There is this guy on strobist (strobist.blogspot.com) who tosses his camera in the air on a timer and gets these really wild shots.
Good luck and take any of the extra booze home with you, you'll need it.