I noticed FB and Gtalk will fail if I have them online on mobile network (on T-Mobile US) and later connected to a Wifi network.

Both of them will show Incorrect Password upon reconnection, other MSN, Yahoo and Skype can reconnect without problem.

The Incorrect Password will not go away even if I keep trying to go online on FB and GTalk.

That problem can be "fixed" if:
1. Connect to mobile network manually.
2. FB and GTalk will go online without problem after swtich back to mobile
3. Go offline on both.
4. Connect back to wifi
5. FB and GTalk will now go online again. However, FB sometimes might ask for the password again.

It seems like you have to sign off on FB and Talk before switching network or some connection states will be messed up to prevent login from the new network?
The FB might occassinally lose the password too.