Keith does offer up a good argument on 24-70 vs 24-105.
For me, I shoot a lot of weddings as a second shooter and the 24-70 NEVER comes off my camera. It is incredibly sharp at ALL apertures.
That being said, I would love to have the 24-105L IS f/4 simply due to the additional 35mm of zoom. The f/2.8 on the 24-70 is really moot with the IS on the 24-105. The 2-stop IS more than makes up for the 2-stop loss of the 24-105.