I doubt the serial port even could reach the 1.5Mbps (may be slightly off, going from memory) of low-speed USB (designed for mice, keyboards, et cetera). Even if it can you'd have to take low-level control of serial. There is a post talking about USB host using USB client found on the Zauruses, has some good info on what it would take if possible.
There are some limited function translation chips that convert USB used by cell-phones to serial usuable by PDAs. Some car mounts have them. I do not know the models of the chips, only that they exist, so I can't look up the exact specs, but from what I can figure I think they implement only enough of USB host to work with a serial to USB chip, and then tranlate it back to RS-232 serial.