'3' have a PrePaid mobile broadband data 'account' option - I'm using this while in Melbourne for ASCILITE2008!
Cost is $10 for the SIM card, $29/month for 2GB data
This is pretty good value & no long term contract required!
AND you can use your own 3G Modem - I'm using a NZ Vodafone 3G Expresscard Modem with the '3" SIM. Only downside is some of the activation process requires the '3' modem software, and it's a pain having to remove cookies to get beyond the 'activation' page each connection :-(