The Prizm's 2.5mm link port is a standard UART, capable of operating at up to 115200 bits per second in normal configurations. See the serial syscalls for usage information in add-ins.
Pinout and voltage levels go here pending further investigation..
When the system PLL is reclocked, this also affects the serial bit rate (which is presumably derived from the peripheral clock). For this reason, serial communication with an over- or under- clocked device will generally not work correctly, unless the other device is similarly adjusted.
If transferring data between two overclocked Prizms, the transfer is noticeably faster.