I'm probably a bit late getting in on this, but can I suggest you not even think about roaming using your Australian Sim card.
I recently had the experience with one of our staff. Over $7K in 3 weeks and believe me the quantity downloaded wasn't high. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
