All the major Australian carriers offer prepaid for the iPad. For best coverage, go with Telstra NextG, then Optus and finally Vodafone/3. Pricing will reflect this as well.
Optus is renowned for image compression, so for the best speed and coverage I suggest Telstra.
Telstra iPad
Optus iPad
Vodafone iPad
Oh and for more reading about iPad prepaid in Oz visit the
Whirlpool Forums...