Obviously this is an old thread, but thought I'd reply in case it is useful.
I wanted to do the same thing, using a serial to usb adapter, that would be picked up by the FTDI driver in linux. I couldn't do this with an original sheevaplug dev kit with ubuntu, because it did not have the ftdi_sio kernel module, and it would not build from source, and I failed in my attempts to update the kernel.
Using the sheevaplug UBIFS kit containing debian squeeze, I was able to do this easily, simply plugging in a cheap usb to serial adapter, it was detected as /dev/ttyUSB0, and can be configured with stty or used as an additional console with getty, by modifying the inittab, and yes works at 9600 baud if specified.
