I own the 70-200mm f/4L IS and it's got a pretty narrow DOF at f/4 on the 5D2, I'm generally stopping down to at least f/5.6 if I'm using strobist gear anyway. Unfortunately I can't speak for the f/2.8 but as Daniel said there will be a narrower DOF and a "larger" bokeh pattern. The quality of bokeh on the f/4 is outstanding as far as I'm concerned and it's a great portrait lens on a FF camera.


Here's an example:


5D Mk II, ISO 100, 1/5sec + 70-200mm f/4L IS @ f/4 and 200mm. Shot on a tripod in natural light.



Quiet time by Ben__Taylor, on Flickr


Cheers, Ben.