Well Jerry, I don't have a 30D, but I use a 5D with the 580 and have been in similar situations in dark churches with the party coming down the aisle during the processional. I believe this is what you may be referring to.
I come from medium format where I used the camera settings on manual with the flash on auto, but there are similarities with digital. What I've done that works out pretty well is to set the Canon on manual, usually f/8 for a modest depth of field to allow for chancy autofocus targetting, 1/30 or 1/40 thereabouts for shutter speed, and the 580 set for auot ETTL. I haven't tried this with AI servo yet, just the one shot (I've only had the 5D since March). But I've been very pleased with the results.
BTW, I'm usually using ISO 800 if it's a pretty dim church. Now here's the caveat: If things are TOO dim, and the camera can't see enough contrast on the target/bridesmaids to happily autofocus, etc. the prefocus LEDs of the 580 will be working overtime painting the subject, trying to target them. You will have to press the shutter a couple of times perhaps, until the camera is happy that it can see what's coming down the aisle.
This is aggravating, but if you leave yourself enough room as you walk backgrounds, you can get some very, very nice shots. Just don't panic and run from the church screaming - it upsets the bridal party, plus they may not let you near the bar later on (just kidding of course). You just have to work at it.... and practice..... and practice.....
- Stu