Hey Sue, firstly the 40D is a great camera & I use one often at weddings.
As most of the other guy's have all said, you will need a flash : and you will need to learn how to use it in manual mode on your Camera!
You may find your 28-135 lens is just not "wide enough" for group shots.
As the 40D is a 1.6 crop your 28-135mm lens will be equivalent to apx 44mm-210mm lens on a full frame camera such as a 5D.!!
This can be a problem at a wedding if there are lots of people, all moving around chatting and the only way you can fit them into frame is to step back. (the old "off the cliff thing!) - frustrating, and then you miss the shot...
A pro friend of mine uses a Sigma 17-70 f2.8-4.5 for weddings, and loves it! They are cheap, cost wise, but great quality!- check out the prices. (and they are also f2.8 at the wide end)
A 17-70mm lens is apx equivalent to 24-105mm on a full frame camera, and this makes it a great "walk around & everyday use lens.
The only other lens you will (or should have) for a wedding IMO is a 70-200mm f2.8.
I use a Canon 70-200 IS 2.8 lens, but Sigma make an un stabilized 70-200 f2.8 which is well worth looking into.
With these two lenses you will have everything covered, from group shots and "randoms" with the 17-70mm, to Bride & groom portraits, and randoms with the 70-200mm. (Borrow a second camera body on the day, or you are asking for "Murphy" to pay a visit!!!)
Now regarding lighting.........(and you will need it !!!)
A Canon 430EX flash will work ok and give you the ETTL when you need "On Camera lighting". But............(there's always a "but" isn't there!!)...- Ok, you have time, learn to use "Off camera lighting". You don't have to spend a fortune at all to do this, for $50 you can buy EBAY flash transmitters/receivers and you will get a fantastic result.
I know this may, (or may not) be over your head at this stage, - so if im telling you to suck eggs, (Aussie slang for "Hey, - I knew that already") then don't chew me out. !!
Do your self a favor and check out this website:
http://strobist.blogspot.com/
Bookmark it and get into the "Lighting 101" lessons.
I have learnt sooooooo much from David Hoby's insight into using cheap off camera lighting, and you will love the results you will get.
I brought two Vivitar HV285 flashes on ebay, for around $80 each! - these have opened a whole new world of professional Off camera lighting for me. How cheap is that!!
All the best, and get into the "Lighting 101 leassons!!"
Cheers,
Mark