Adam's guide for power travellers got me to thinking about which Australian airports have free Wi-Fi, so here's a list:
Free Wi-Fi provided by ISPs:
Adelaide Airport (Internode)
Darwin Airport (Internode)
Newcastle Airport (Internode)
Albion Park Aiport, Illawarra (Fish Telecom)
The list of free Wi-Fi spots isn't very comprehensive, so you may need to pony up if you want to get online.
Paid Wi-Fi options
Like anything you buy at airports, Wi-Fi will be pricey. Optus offers Wi-Fi at Sydney's T1 and T2 terminals, and at Brisbane airport. You can buy a monthly subscription or pay $13.20 to set up a casual account which costs $12.10 per hour.
Your other option is an airline lounge. Membership is pricey, but frequent fliers might appreciate that internet access is included. Virgin Blue's member Lounge costs $548 to join, ($349 per year after that) while Qantas Club costs costs $775 for a year (or 150,000 frequent flyer points). However, Virgin Blue does offer a $30 casual rate if you have a Virgin Blue boarding pass, so you can access the Wi-Fi in their Brisbane, Sydney, Melbourne or Adelaide lounges.
Qantas covers more airports - they offer wireless at Adelaide, Alice Springs, Brisbane, Cairns, Canberra, Darwin, Gold Coast, Hobart, Melbourne, Perth, Sydney T1 and T2 and Townsville.
That's about all I've been able to discover about Wi-Fi options at Australian airports. I do know that McDonalds, Starbucks and Gloria Jeans often offer Telstra Wi-Fi at their stores, so it's possible that they may have an airport outlet offering internet connectivity. If you know of other free or paid Wi-Fi spots at Australian airports, please share in comments!