A friend pointed this out. FM transmitter plus USB host support plus cigarette lighter-power (plus your USB flash drive) equals a cheap little portable in-car solution.

Of course, the fact that I can't find any web site about it besides the Wal-Mart one doesn't make me feel confident, but, hell, Wal-Mart's pretty good about returns (their slave labor must like opened products). And it's less than $30 anyway.