WiFi is a bit harder than other wireless interfaces, but doable.
I wouldn't recommend attempting to use a USB wifi stick, writing a USB host driver is well beyond the scope of a fun project.
Lantronix -
http://www.lantronix.com/device-netw.../wireless.html is the bet I've found for an easy embedded WiFi solution.
I would strongly recommend considering other wireless methods, as they will be significantly cheaper/easier. Also, please consider upgrading the Stamp to a more modern option, it will make things easier in the long run.
Have fun!