Here is what I had to do to get this working.
1) Put version 4.2.1.66 on my T-Mobile 8100 (Instructions
here)
2) Set the TCP options on the 8100 to blank username and password and wap.voicestream.com as the APN
3) On my TomTom 510 go to traffic, which starts the Plus setup wizard. (This assumes it was already paired for voice.)
4) I chose T-Mobile US IV and walked through choosing the defaul options (This will fail.)
5) Once the setup fails select manual and replace internet3.voicestream.com with wap.voicestream.com in both places that it appears. Don't forget the "
It successfully connected!!!