I recently got a 9800 on loan from someone also with optus after my desire z broke.
My desire z used networking very easily. So I know it is nothing wrong with that.

I have the 3G logo on the top right hand side of the screen. Yet if I try and use the browser I get "Unable to connect to the internet". and using the other apps I get a message which essentially is saying it can not use the network.

I have reset the phone made sure my network settings were set to the right provider and I am out of ideas.

Though I am more motivated to get my desire z fixed.