I'd be willing to bet with a little custom circuitry you could patch together a link with the tether port and some XBees from SparkFun.
If I recall correctly (and to be honest, I'm pretty sure I'm not recalling correctly, but somewhere in the vicinity of correctly) the tether port is basic RS-232 serial, with the wiring a little goofed up (I remember the tether cable being a bit different, I think it swapped TX and RX) and some provisions for carrying power from the RC to the OI.
EDIT: Based on Jasons comments below I'm adding this warning!!
Do
not connect the tether or radio ports on either end of the IFI module directly to your computer or any other device. They're carrying various amounts of additional power/wackiness to run the system that could toast something that's not expecting it!
I'd probably use a couple of these:
http://www.sparkfun.com/products/9111 to hold a couple of these:
http://www.sparkfun.com/products/10414
And you may just need to whip up some adapters, and of course, the modules would have to be configured with the XBee software on the computer (this is easy though). Might need to do some trickery to provide power to the units as well (OI side is easy, use a 5VDC wall adapter, the robot is a bit trickier, but should still be doable fairly easily).
Hmm, this could be fun, if you're interested in doing this project let me know, I can provide some more detailed help.
Matt