UPDATE:
I'm dumb. It's a possum. &ugly.
* * *
I took this photo today, January 12, 2010 at Winfield Mounds in the DuPage County Forest Preserve near Wheaton, IL.
It's not a beaver, raccoon, skunk, or porcupine (as far as I can tell). I'm sure it's something really simple– I just can't figure it out.
It's got the shape of a raccoon and the tail of a rat.