Once you have the kernel built for peripheral mode, getting the gumstix to emulate a serial port is pretty straight forward, just load the g_serial module. This will create a device called /dev/ttyGS0.
modprobe g_serial cat /dev/random > /dev/ttyGS0
I could get about 3.5 Kilobytes a second over the link.
You can also use g_multi to get serial and mass storage at the same time:
modprobe g_multi file=/file_store_data