FWIW, looks like a couple amazon merchants still have it listed for ~$35:
http://www.amazon.com/Xbox-Wireless-Gaming-Receiver-Windows-Pc/dp/B000HZFCT2/ref=sr_1_3?s=videogames&ie=UTF8&qid=1288447669&sr=1-3
Yeah, makes this a no brainer if you don't have a receiver yet and want to use this on...