For a wedding you really need more focal length coverage than up to 55mm (although it can be done), you really need 2 cameras, 2 flashes etc etc.... What if your camera stops working on the day? (I've had a friend this happened to - she called me in a panic). my advice is do not gamble with people's big day - it will bite you big style!
I'm not a wedding pro as such. I book a few a year through word of mouth but decided that I neded the right gear. 2 bodies (one borrowed), 2 flashes, a range of lenses from 10-200 (although I can go to 420mm if I need to) with large apertures (mostly f2.8 and faster).
You need fast lenses and the 17-55 IS is a great choice but what if you go to a church and the minister asks you to stay at the back with no flash (this happens)? The 17-55 is not going to give you those close in shots you need of the exchange of rings or the kiss etc. Maybe a 100 f2 might be of use
70-200 f2.8L IS would be the only real choice for a wedding pro using Canon gear.
i'd suggest not being a main photographer until you have more equipment and experience. When you shoot as a second photographer you'll realise the complex issues of wedding photography.