People who photograph weddings are a brave lot. :) On 95% of all the stuff I shoot with the 10D (with the exception of the studio shoots) I use the 550EX. It's a fast recycling, bright flash. I almost always use a lumiquest softbox on the flash (unless I'm shooting from more than 10 feet away from the subject). I have found that it can tend to underexpose if I'm not paying attention to what I'm shooting and my metereing mode (for example, shooting where the background is bright and not switching out of evaluative is just asking for trouble). I've never shot a wedding before, but I would probably rent some L glass. Probably something like the 24-70mm f/2.8L lens and the 70-200mm f/2.8L IS lens. Might even bring along a 16-35mm F/2.8L for wide group shots. If you're not having to shoot from the back of the church, the 24-70mm is a fantastic lens. Loved using it at the Hot Import Nights car show earlier this month (Seriously low light event). With the low noise of the 10D, I would feel very comfortable shooting a wedding at ISO 200 or 400. At ISO 400 and 2.8L glass, I'd think the flash would almost start firing as fill. But again, don't shoot weddings so I have no real idea.