I currently use the Lantronix XPORT with a whole bunch of PIC- based projects.

The Lantronix has a pin (CP2) that can be programmed to be a "carrier detect" pin. This pin goes "low" whenever a telnet session is in progress. I use that signal to "hijack" the RS232 port (using a 74HC125). That way, I get RS232 functionality when there is no active Ethernet session. It all works automatically with no manual switching.
The best part about the XPORT is that you don't have to deal with protocol stacks or the processor overhead they it would take. The PIC just "talks" RS232 at 9600 baud.
One downside - to change the IP address (or any other parameter) of the Ethernet port from the "serial side", you have to write what Lantronix calls Setup Records. These are 128 or 256 byte strings of Intel Hex. You have to write the entire record if you want to change only one variable.

I can give details if anyone needs them.

Charles Linquist