Just seen that the preorder for Aliens is up on Steam and once again Australians are being ripped off.
Current US Steam price for the preorder is US$49.99, Australian Steam price is US$79.95.
So avoiding Steam I was able to find a Preorder for US$38.49 at GetGames.
Aliens Colonial Marines - Download Games - Get Games
I have bought with them before (Civ 5 GOTY), when the game comes out you will be provided with a keycode that can be entered into Steam to activate the game on your Steam account.