Devin said:
Just a thought, but if you have a Laptop with a Wireless card you could bridge the connection with your Xbox 360 in order to give it access to the internet. The only downfall is that the laptop has to stay connected to your Xbox 360 via ethernet cable.
You could do the same thing with an OpenWRT and/or DD-WRT compatible router in client mode. That tends to work a lot better stability wise than doing it by bridging connections in Windows. I've used it a few times to connect different network segments to each other over a wireless link, and once it's configured, you never have to touch it again.
As for hardware, I'd go for a WRT54GL, it's tried and true. You should be able to get one of those dirt cheap at Ebay, or if you rather want a new one, that shouldn't set you back too much either. They're ~350 SEK (~50 USD) in Sweden, and everything is a lot more expensive here than in the US. In other case, there are plenty of other alternatives:
DD-WRT compatible devices,
OpenWRT compatible devices. Perhaps you already have something you can use, collecting dust somewhere?
Installing one of the aforementioned third party firmwares isn't harder than downloading the firmware image for your model, and installing it from the router's web interface, just as you would with an official firmware update. There are a few exceptions where the procedure is a bit more complicated, but those are rare. Once you got the firmware installed, connect the Xbox to the router by ethernet, set the router's wifi to client mode and configure the networking from the web interface, and then you're good to go. Google for a guide if you can't figure out how to go about the configuration yourself, I'm sure there are plenty of those around