eWeek reports that WPA2 support under Windows XP SP2 lacks 802.1X supplicant upgrade: Microsoft's WPA2 support should probably have included the four mandatory supplicant secured EAP types required under Wi-Fi Alliance certification standards for WPA2. I say probably because the new secured EAP types were added after the WPA2 certification standard was initially set.
What this means in practical terms is that you cannot use 802.1X with anything but EAP-TLS with Microsoft's native supplicant. You'll need third-party software. Microsoft's WPA2 update boils down to just WPA2-PSK (Preshared Key) support for networks that really don't need CCMP keys using AES if they're just going to all share the same key!
Update: A colleague tells me that WPA2 support for Microsoft's flavor of PEAP is included with their native supplicant. Since that's now the dominant secured EAP flavor outside of installations in which companies particularly work hard to use another method or have all Cisco gear, that's really the most important piece.