Is it true that StarCraft 2 is separated into different regional sections (America, Europe, and Asia) for multiplayer gaming, meaning that if I purchase SC2 in Asia (Hong Kong) on a holiday, that I will be unable to play with the people in Europe when I return back?
Currently, you will be unable to play with people in another region.
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:
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: