@Beck -- Cooper's Hawks and Sharp-shinned Hawks look very, very similar but for size and some minor details (such as the shape of the tail at the bottom). Size alone is a tricky way to distinguish, because the bigger SSH's can be as big as the smaller CH's. This one was about the size of a crow, so it's a pretty good bet to be a CH, especially paired with the way his belly stripes fade out toward the bottom, so I'm reasonably confident in the ID -- but I'm not very good with the hawk IDing yet in general, hence the caveat.
Here's a page that helped immensely with some of those little details (because my usual book, the Nat'l Geographic Birds of N. America, was failing me): http://www.birds.cornell.edu/pfw/AboutBirdsandFeeding/accipiterIDtable.htm
Also, I've found any juvenile bird will give more trouble than the adults (not just hawks), simply because they tend to all be browns and creams and greys of some sort, with many fewer distinct identifying marks. It's like they're trying to hide or something. :-)