Well I had DUN working on Cingular Media Net for a while then it seemed to break.

Tried everything. Hard reset. Treo Phone support they even didn't charge me .. no luck!

What did fix it was just deleting the string in the advanced modem settings. The one that starts AT+ . I'd check it several times for typos an was going to re-enter it from scratch just to be sure. After I deleted it I thought I'd try it blank just to see if the behaviour changed. It just came right up!

Hope this helps safe someone else some time!