As a defence for the 5D, it can do sports. Sure, the AF and FPS is a bit slow, but it's definitely capable of producing wonderful shots. I did quite a bit of sport coverage using just the 5D/70-200. It really puts the emphasis on anticipation and the idea of anti-spray/pray. But undoubtedly, the 1DMkII will do much better.
Now as a defence for the 1DMkII, though marketed as a sports camera, IMO, is an extremely versatile camera. In comparison to newer cameras, the high ISO isn't great. But quite honestly, it is not bad...at all. To add, the APS-H sensor is a rather good compromise. Switching between my 5D and 1DMkII, the smaller viewfinder does not bother me at all. Looking through a 20D on the other hand...
So those are my opinions. As for whether or not losing MP and full frame is worth it, it's a question only you can answer. Do you find yourself having to crop frequently? Do you need the fast FPS and AF? Do you need the build quality (which BTW is incredible)? Do you print your work frequently or simply post to web? If so, how large do you print? Questions like that will help determine whether or not it'll be worth it to you. A sports shooter will need the AF of a 1-series. A landscape shooter will need the MP of the 5D.
Now, my story. I am an "everything" shooter. From landscapes to sports to portraits, I shoot them. I shot the 5D exclusively for nearly 2 years. I, too, saw 1DMkII for about that same price. Decided, "I think it'll be fine". I sold my 5D and picked up the 1DMkII. And truthfully, I really like the 1DMkII. It still gives me superb results, but a little part of me is still attached to the 5D. After a couple months of owning just the 1DMkII, I bought another 5D as a second body. Now, I have the best of both worlds and could not be happier (though I wouldn't mind MkII/III versions of both cameras now...)
Hope this helps!
5D Mk II | EF 20 f/2.8 | EF 50 f/1.2L | Yashinon 50 f/1.7 | Samyang 85 f/1.4 | 580EX | 430EX II
Michael Chu