Did a search and couldn't find it, so hope it's not a repost. Woot.com has the Linksys Wireless N gaming adapter for $32.99 + $5 ship. Should work great with the Xbox 360.
Really interested in this. Getting kinda tired of running cable all over the house. The comments from people who own one say it works pretty well. Plus it works with ANY ethernet device. Not just ethernet enabled gaming systems.