RS-232 cameras exist, but are too slow to be used as a rear-view cam. Most RS-232 cameras I've come across take a picture then transmit it, which takes two to five seconds. Not really helpfull when you want to make sure that there isn't someone or a moving car in your path. I'm not sure of a vendor that sells them in a small quantity, as most are intended for industrial parts recognition.
Last I heard, the Gumstix developers were working on a USB host expansion, perhaps waiting for it, or developing it yourself (a post on the gumstix forums has most info you'd need) would be better.
Hmm... the Etherstix would work if you got a network camera.