While the Xbox doesn't have any built-in wireless technology, there are plenty of devices out there that can be hooked up to the console to get it on your wireless home network. These devices connect to the Xbox via Ethernet, and then act as a wireless bridge connecting the Xbox to your router.
Most of the major network gear manufacturers out there such as Belkin, Linksys, Netgear, SMC and Netcomm all have wireless gaming adapters in their product line-ups. If you want to read up on some of your options, check out our reviews of the Linksys WET54G Wireless-G Ethernet bridge and the SMC EZ Connect G 2870W Wireless Ethernet Bridge.
Wireless on-line gaming will be a little easier with the upcoming Xbox 360. While the next generation console won't come with wireless as standard, it will sport an expansion slot that you can plug a Wi-Fi card into.