The good news is you don't have to drop $50 on Microsoft's offensively overpriced Wi-Fi adaptor. The bad news is you'll need a laptop with W-Fi to do it.
Control Panel – Network Connections (In Windows XP) or Network & Sharing Center – Manage Network Connections (in Vista/Win 7). Select the Local Area Connection and the Wireless Network Connection at once, then right-click and hit 'bridge connections'.
Disconnect then reconnect to your wireless network, run a network cable from the laptop's Ethernet port to the 360's, and you should be good to go.
Unfortunately, you may have to remove the bridge (repeat the above process and you'll see the option) whenever you want to browse the net with the laptop.
