You want an Ethernet client. IIRC. the Linksys WRT54G router will do what you want, but maybe only if your AP is Linksys AP.
The Linksys WAP54G V2 claims to work as a client to (lessee) only another WAP54G, though maybe the later hardware revisions will work with other hardware.[Are you really sure you want to do this? IMNSHO, wired Ethernet is far faster and more reliable (to say nothing of more secure) than wireless, and it's probably cheaper to run wires than build out a whole wireless infrastructure...]