Do you mean the MS XBox 360 wireless adapter doesn't support WPA2? The XBox 360 itself is neutral on the issue. Whether support for WPA2 exists depends on the attached wireless adapter.
In my house, the DIR-655 works connects at n rates in the Auto(WPA/WPA2) setting, using the 1.05 firmware. Connection rates die at the the WPA only setting.
The DGL-3420 game adapter supports WPA(AES), but I don't recall if it supports WPA2. Unfortunately, the DGL-3420 does not play nicely with the DIR-655 in any mode, and will knock your network to sub-802.11b speeds (intolerable).
For now, use a wireless-g access point (or an old wireless g router configured as an access point) as explained above. Then, wait for a relatively inexpensive wireless n game adapter (I have a feeling that the forthcoming n media bridge will be considerably more costly than a game adapter), or a firmware update (I hope) for either the DIR-655 or the DGL-3420 that will allow them to play nicely together. Of course, having a wireless g device will slow your network to wireless g speeds, but unplugging the game adapter when not in use should be a simple and relatively easy compromise.