I've shot a few weddings now...a couple with the XSi and a few with the T1i. Each time I used a 580exII and several different lenses.

For group portraits, a Sigma 24-70mm f2.8 is what I use. Like someone else said, if you can get the flash off the camera and give it some directional light (usually through an umbrella), that will look the best. A good compromise is to buy a big reflector (like 42") and use that to bounce the flash off of. Conditions/style vary, but I like to have the background slightly darker than the subjects, so underexpose by a stop or two and let the flash brighten the subjects (not sure if that makes sense or not).

For 1 or 2 people, there's nothing like the Sigma 50mm f1.4. Awesome detail, great bokeh. Really good low-light lens for the reception too. I like to keep the appeture pretty wide, usually around f2.0. It's a very narrow DOF, but the results are very nice.

For the ceremony, it depends. I usually hang in the back of the church with my Sigma 70-200mm f2.8...shoot at f2.8 and balance the shutter speed/ISO for lowest noise possible and no blur. Use a tripod if you have to. If you can sneak around with a 24-70 and/or use a flash, you might get a little closer...ask the pastor/preacher/random-ordained-guy for the official rules.

For on-camera flash, bouce it off walls, ceilings...anything that's pretty white. I don't like omni-bounces, because the light looks a little flat. Shoot RAW so you can correct the WB later (churches and reception halls have screwy lighting, plus you might get off colors due to bouncing off something non-white). Use ETTL mode and you should be fine.