Actually you only need an AP if your going to use a workgroup bridge like the WET11. The parent device would need to not be in bridge mode in fact. The WET11 is a client device, like a PC card for a laptop. It just is a bit more advanced and has an RJ-45 port that it passes data to.
Any 802.11b Router/AP combo will do if you want to use the WET11 on your Xbox. If the Linksys literature called the AP side of the equation a bridge, then you just have to consider the source. Bridges do not communicate with wireless clients, only other bridges in a point to point, or Point to multipoint setup. The WET11 is a workgroup bridge, not a standard wireless bridge. It can get confusing, but just get yourself an AP, or router/ap combo if you need the routing functions, along with the wet11 and a patch cable, and you'll be fine.