Last October, THQ decided to block Oz and NZ users from purchasing it games via Valve's Steam. Although the publisher was unwilling to provide a reason for the block, the working theory was that local users could purchase the company's games via the service for significantly less than their price at brick and mortar stores, thanks to the weak US dollar.
Rather than increase the price and face our collective wrath, THQ decided to remove its catalogue instead. But like I said, it's just a theory.
It looks like the recently released Frontlines: Fuel of War slipped through the block - at least for a while. It was briefly available for $US 50 on Steam for Australian and New Zealand users, but it looks like it's been taken down as of today. THQ's local presence couldn't really comment as this sort of thing is out of its hands.
If I had to guess, I'd say that Frontlines was never meant to be available to us from Steam. THQ/Valve has just corrected the error... much to our chagrin. Good news is that if you did purchase it while it was up, it's still playable.
Uhh..I live in Australia, where did Frontlines go? [Steam Powered forums, thanks Lev Arris]