As someone who moved from the 10D to the 1Ds, I can tell you that you will see a huge difference in image quality, but depending on your cropping and output needs that may be something you can live with.
IMHO, one of the biggest problems in making the move you propose is going from a full-view sensor to a 1.6 crop factor. If, as I am, you have years of 35mm experience, switching gears to rethink every shot in terms of cropping is a big distraction. Everything you have come to rely on as instictive when choosing a lens will completely change. Others may adapt to this quickly--I never did, and ended up selling the 10D.
Also be sure to do a little touchy-feely on a 10D before buying. I found the user interface of the 10D to be radically different from the 1Ds, and think the 1Ds is much more intuitive.
So my only point, and it is just my opinion, is that there is much more to consider than just image quality in determining if you really want to downsize. I can not imagine being happy moving from my 1Ds to a 10D, but your mileage may vary.