I have always used my X51V running WM5 with WPA/PSK-TKIP and 802.11b. Sucessfully, I might add. My home access point actually runs in b/g mixed mode.
Could be that there is a mismatch between the network and your device. A couple of possibilities include:
- TKIP vs AES
- 802.11b vs 802.11g
You can fake out the radio on the X51V to connect to 802.11g, even though the device is actually only 802.11b via tweaks.