Actually, a probable possum. The cats alerted us to something sneaking around the woodpile last night, and it turned out to a giant-rat-like critter. After our initial hopes of an ROUS, some poking around in google images lead us to the possum hypothesis.
As well as giving me the chance to say 'possum' a lot, which is a treat in itself, it was also another in the list of cool American critters I've been lucky enough to see. It joins coyotes, tarantulas, scorpions, bobcats, rattle snakes, king snakes, racoons and black widow spiders. The black widows are also prone to hanging around our house, which adds a certain edge of danger to walking to the hot tub at night!