Tentatively identified as a Cooper's Hawk.

There are some more pictures of this fine fellow in this gallery: http://www.pbase.com/corvidae/birds_hawks
It actually sat in the tree for quite a long while, and was stretching or preening in this shot, which I took shortly after the sun came up. It looked very contented.
I posted this image mainly because it was a bit unusual and interesting.

It went off hunting a couple of times during the day and came back to the same spot in the tree.

Here's a side view of him (her?) later in the afternoon.
The picture's not as good, but you can see it's back: