Starcraft – Is StarCraft 2 Separated into Regional Gaming for Multiplayer

Short Answer

Currently, you will be unable to play with people in another region.

Long Answer

The serial numbers for StarCraft are keyed to specific regions. When you register the key, it locks you into playing people in that region. You cannot play with people in a region outside of the region for which your StarCraft 2 serial number was created.

It has been stated on the official forums that it is possible to change the region under which your game is assigned via phone support:

Under certain circumstances, we may be able to change the region of your StarCraft II license.

Due to security reasons, however, this is not something that we're able to assist with through e-mail correspondance. If you would like to inquire about this, we recommend contacting our phone support, as they have the proper tools to perform that type of procedure. There are some limitations that come with this, which will be explained further once you get in touch with our phone support.

Note, however, how this changes your region. It doesn't allow you to play in two regions at once. This related question states that Blizzard is looking to change this in the future so that you can play in multiple regions:

Unfortunately, there are a multitude of challenges we have to overcome due to the unique regional account and billing options that didn't exist in the past. But those hurdles aren't insurmountable, and we are looking into solutions that will allow interested players to obtain access to other regional versions without having to buy another full copy of the game. Those solutions are something we're currently planning to have available through Battle.net Account Management within the first few months of StarCraft II's release.

Before that solution is implemented though, you're correct in that you'd need to purchase a US copy of the game on launch day to play in the US region.