IOS4 for the iPad is not exactly the same as for the iPhone 4 (although they use the same code base), so there is extra work involved. Apple engineering is not that large, and they have had their hands full with the iPhone 4 recently.
Let's be honest, we would rather have a thoroughly tested IOS4 with as few problems as possible than something rushed out.
Although I am looking forward to multi-tasking, folders will make a big difference to me. My iPad is filling up with apps and I end up flicking between screens all the time to find them!