If you have enough light, try shooting ambient light. Your 17-85IS should be able to handle pretty low shutter speeds with ease! At 17mm, you can probably get away with 1/4 sec. At 85mm, you can get away with 1/20 sec.
I prefer shooting ambient whenever I can, because flash ruins a scene. Unless you use the flash really skilfully of course.
Here is where your 20D has a big advantage over other camera systems - high ISO noise is really very well controlled. The pro photographer was using a camera from the "other" camp and he saw my images (see below). He later mentioned that for quite a while he was thinking of jumping ship and my images helped to convince him. I could bump my ISO to 3200 and know that my images would turn out OK.
So this is what I recommend: set your camera to M (or Tv as Petal suggested), set ISO to 800-3200 depending on how much light you have, then set shutter speed at 1/focal length (minus 2 stops in your case, since you have IS). I then either vary the ISO or the aperture to my liking.
If I have to use flash, I prefer a combination of direct + bounce. The main problem in events is that the flash has a white balance of 5500 Kelvin, and events are usually lit with tungsten lighting. So it can sometimes be tricky mixing flash in with ambient and requires skilful use of the flash.
I don't like bouncing flash from the ceiling because it gives nasty nose and chin shadows. Whenever you can, try to bounce flash from walls. I have heard good reports of the Gary Fong lightsphere and I want to give that a try ... so that's yet another option for you.
IN GENERAL you want your flash dialled in -1 eV to make it look as natural as possible and not overpower the scene.
Here are a couple of images, captured on a 20D with an EF 24-70/2.8L. Both ambient, handheld, no flash.