The below wireless wifi IM gizmo appears to sell for ~$40. Not sure how hackable it might be for wireless robot control or data transfer.
edit:
apparently there has been a good bit of hacking on this device in the past. According to some of the info in the below links, linux can be loaded and run on the unit, as well as adding a serial port and extra memory.
openzipit.org/
zipitwireless.com/linux.html
aibohack.com/zipit/index.html
tech.groups.yahoo.com/group/zipitwireless/