This works flawlessly with my Cingular 2125. In fact, I am writing this using my Cingular 2125 tethered as a bluetooth modem to a Powerbook running the lastest version of OS X. Haven't been able to get this working before.
Now I've got to spend a little time getting this working on my PC.
Because the 2125 supports EDGE, the speed is much more acceptable than with my previous phone (a Sony Ericsson T616). Browsing the web is now practical.