Quote Originally Posted by Sean Setters

I've really become accustomed to having wide apertures and image stabilization available, and it's difficult for me to let go of either one. On Saturday I'll be shooting a wedding, and I'll probably be glad to have both.
If that was your concern, then you can rest easy. The 24-105 has a *wider* aperture than the 17-55 at every equivalent field of view and perspective. For example, the aperture on the 17mm at f/2.8 is 6mm. The L at 27mm f/4 is 7mm: 17% wider aperture. The larger front element of the L points toward this fact too.

The focal ratio (not aperture) is one stop narrower (f/4 vs f/2.8), so the light intensity per area is twice as dim. However, the total amount of light is more than double, thanks to the much larger area and wider aperture. So you can up the ISO or use -1 EC to get the same shutter speed as with f/2.8 and still collect a half-stop more light in total. The resulting image will be superior to the 17mm on the 30D, and only slightly better than on the 50D.

I hope that help.