quick update:

instead of un-installing/re-installing everything, I tried rebooting the computer.

When I tried again, I received an error (762?) saying the computer and the remote computer could not agree on a PPP. I remembered that one setting people were playing with was "use IP header compression" which is under the heading "PPP Link." (Settings->Network Connections->T-Mobile GPRS->Properties->Networking->Internet Protocol(TCP/IP)->Properties->Advanced)

I unchecked this box. Back to Error 50. Rebooted again. Now it works but only till I disconnect. then I have to reboot again.

Any thoughts?

p.s. I am with T-Mobile on the EDGE network.