My main problem with the 70-300L, is there is no panning mode for the IS.
If you don't need to pan while shooting, definitely consider the 70-300L.
I thought there was? (I can go check the user manual when I find it later and scan it). It's got the 'IS Mode' switch 1-2, which is normally 1 for normal IS and 2 for panning. What it doesn't have is the Mode 3 which only turns the IS on when you press the shutter (which is more useful for tracking birds and cheetahs), that's only on the $10k mkII über-teles.
I got this shot a few weeks ago, at the Tour Down Under. 7D, Tv 1/400s, Autoiso100, f/7.1, 70mm (so not too far away from me), standard processing and no cropping. Bike and her face is perfectly in focus, background is pan-blurred left to right.