actually I just read the Jango website and it looks different to the other Optus phone I used. I looked up the GPRS settings and found these look more like what I remember.

Operator: - Optus
GPRS APN: - internet
Username: - {blank}
Password: - {blank}
DNS: - 202.139.83.3, 192.65.91.129

Not sure if you need to put the DNS into the N900 settings though.