I have not gotten around to looking up exactly what this thing is yet, although it resembles photos I've seen of hairstreak butterfly larvae.

UPDATE June 6,2006: I found this thing at bugguide.net - it's a Callophrys henrici or "Henry's Elfin". Apparently the larvae are usually green, but turn red shortly before pupating. They feed on the leaves of redbud trees - it may have climbed up the side of my house after falling out of the elderly redbud tree that overhangs my patio.

More info and photos (including adults) here: http://bugguide.net/node/view/3570/bgpage