I don't know why you'd need any more lenses than what you have. Chances are you'll have time to swap, so I'd take more than one, for sure.
Having said that, I've not had the opportunity to bring my DSLR to a wedding, and I'm not sure what would be optimal. When I had P&S cameras, it was about impossible to get anything good -- too dim.
Even with photos where many were marred from too much camera shake, those involved with the wedding itself are usually too busy to take photos, and are happy to take whatever they can get. So, I say, with a DSLR and f2.8 lenses, you should be able to get things much better than that. I'm sure people will be happy to get additional memories of the event.
With a DSLR, I would use higher ISO settings without worrying that it would be too noisy. (ISO 400 or 800 with an old P&S digital camera was a mess!)
What I'm curious to know is whether wider angles are more important, or perhaps something closer to the 100mm range for portraits? My 28-105 is a tad soft -- maybe that'd be good for portraits.
I also wonder if people have a strategy.. do you try to mimic the typical wedding photography with group photos and staged portraits? I have tended to assume that the pro photographer can handle that, and try to get something more candid. Although, you cannot guarantee that the pro photos will actually come through OK, so a backup is not a bad idea.
