Telstra BigPond has an international roaming system. Although it works out pretty expensive, their roaming seems to offer a wide range of countries.
Go to
www.telstra.com and doa search on "international roaming" for further details.
I cannot comment on their roaming service, as I have not used it though. I use their cable service and apart from around 2 severe server problems per month until a few months ago (it seems to have been OK lately), everything seems quite good, and I do not have any intention of changing my ISP right now.
I believe AT & T's roaming is around the same surcharge as Bigpond's, but I heard that the counties and locations covered by their roaming arrangement are not quite as wide. You should be able to find out more at
www.attglobal.net. The people I know who use AT & T say that it's OK but the connection speed is usually low.
I tried AOL for a month and chopped them as I did not like the interface and did not like the slow speed.