Yes Paul, as Mister Octane says, it is a remarkable lens ... and thanks for you kind remarks ...
Optically you would get pretty much identical results with the 400mm prime but the zoom gives you the option of zooming out a bit if you're too close.
When I used the 400mm prime hand-held I found I needed plenty of light in order to get short exposure times. The IS in the zoom makes it a bit more forgiving in that respect.
Don't stress too much about camera bodies. They are after all just a light-proof box. Sure some have a few more bells and whistles but the best investment you can make is in good glass. It took me a long time and a fair amount of wasted money to reach that conclusion!
Also, consider going along to a local camera club meeting. I learned more in a short time at our local club than I ever did from books or the Internet.
Here's another pic with the 100-400 ...
http://www.flickr.com/photos/andy_keir/2031056165/