pretty sure it isn't that, spent a whole day and bought simcards for every network in Australia and replaced my simcard. I have friends on optus and virgin with n900s (same network as mine) and they work fine. I even got a second optus simcard, and I got my simcard replaced, same issue.

My phone is stuck on 2g, with both optus and Vodaphone, which are both UMTS 900/2100 It only gets 3G with telstra, which is UMTS 850/2100...