An xbox360 wireless card is $100, and it's theoretically the only wireless card that works with an xbox, so if you want wireless internet, you have to buy that card. I guess the idea is to sell the console cheap (for sufficiently large values of "cheap") and then gouge customers on proprietary cables and addons afterward. Microsoft isn't the only company that does this, by far. (Not nearly as bad as $20 for 8MB worth of PS2 memory card. Ughhhh.)
Fortunately, if you have a spare Linksys router lying around (as I do) you can throw DD-WRT on it, put it into Client Mode, connect your xbox to the router via a bit of ethernet cable, and there you go. I can also plug my aging desktop machine (sans wireless card) into the same router, and two other devices if I can find any.
Installing DD-WRT was surprisingly straightforward if you take the time to read through the wiki instructions first very carefully. A bit of healthy paranoia of turning your hardware into a brick goes a long way.
15 minutes, $100+ savings. Thanks again, Linux and open-source community.