The good... you have one nice body and one nice lens. Alot of people ask similar questions with entry level / beginner DSLRs. So thats a good start.
The 24-70 should be enough for standard stuff, although you might want to rent a telephoto lens to get in closer. It is their day, not yours, so you need a longer lens to grab those images while staying out of the way.
As mentioned above, a backup for everything is a must! You pretty much have no do-overs, so if your camera clunks out, you are screwed.
How about flash? You have one listed, do you know how to use it? Diffuser, bouncing? Extra batteries and another flash.
One other thing, don't take pictures of center pieces with a cool background. Like mentioned earlier, take pics of bride and groom.
I 100% disagree, worse advise I've read in a while. On what basis do you give this advice? Take pictures of centre pieces. How many times has a bride ensured I took pics of the centre pieces. Many brides do them themselves or spend time thinking of the right one. It is an essential piece of the wedding table you need to document. Take images of that.
I am not 100% of the time with the camera in the B&G's face. I take images of the wedding party and some of guests. Its about telling a story, and a story includes not just the main actors but a supporting cast and the set as well.
Don't take thousands of images of things, but a few good ones. The cake (without the B&G in the image), the center pieces, the empty room, place cards...these are little things that make a nice album.
One other thing for the "pros" out there. Would you ever consider shooting a wedding with an XSI only. I have one and want to feel better about my purchase.
You are obviously pro enough to give advice to someone wanting to shoot, so you should know the answer to your own question :er:
While I am not a pro, I have shot weddings. I shot my first wedding with an XSI. Worst experience ever. I had a 28-75 f/2.8, 50 f/1.8 and a cheap telephoto. I also had a 430EXII flash and didnt know how to use it.
I struggled to get images in the church. I struggled to get images at the reception. I did get some good images outdoors posing the couple when the light was good.
Like this one
Emily & Simon | Flickr - Photo Sharing!
But I was not happy with what I did overall. My equipment severely limited my work.
On the positive side, I did this for a friend who didn't have money for a photographer. When I had "the talk" with her, I told her she should hire a pro to get her the best memories possible. She told me it was either me or relying on friends with P&S cameras. I didn't accept any money and she got better images than if I wasn't there.
So the answer....no. An XSI isn't enough to shoot a wedding, even with some 2.8 lenses. You are severely limited in your usable ISO (I didn't like results over ISO 400).
What do I shoot with now? A 7D and a 50D as backup. 70-200 2.8 IS, 50 1.4, 28-75 2.8, 580 EXII and 430 EXII. And even then, I'm saving for a full frame