First off, let me say that Blizzard's region locking is fucking gay. I have a few mates in the states and not being able to play the SAME GAME against them sucks.
Since patch 1.1 of Starcraft 2, Blizzard was "kind enough" to allow Aussie (and Asian) SC2 players to play against the fat North Americans.
To enable it, you must first log into http://www.battle.net with your SEA account. Under "Your Game Accounts", click on "StarCraft II: Wings of Liberty [SCII-1] North America (NA)" to add the game to your profile.
Now start up the game.
Before you log in, click on the region button on the left side of the screen. Select a region and then log in as per usual.
Let's all hope this sort of region locking gayness won't ruin the experience of Diablo III!
If you can't see it, you're probably using the "Remember me" option. Click on the little arrow next to your account name to clear it and the "Region" button will appear.
Happy Halloween! =)