Blizzard has announced Global Play for Diablo III, which will allow you to join up with friends around the world to kill monsters. You'll have to create a character in the proper region, though, because items, friends lists and characters are region-locked. Still, you can hop out of your home region and head to another if, for instance, you live in the United States and want to play with someone from South Korea.
Blizzard is segmenting the globe into three parts:
The Americas, which includes the United States, Canada, Latin America, Australia, New Zealand, and Southeast Asia.
Europe, which includes the European Union, Eastern Europe, Russia, Africa, and Middle Eastern countries like Israel and the United Arab Emirates.
Asia, which includes South Korea, the regions of Taiwan, Hong Kong, and Macau.
Blizzard also specified that the real-money auction house where you can buy and sell items will only be active for your home region. If you're particularly interested in using the real-money auction house, it will be accessible one week after Diablo III's May 15 launch. Prior to that, you'll still have access to the auction house that uses Blizzard's in-game currency, gold.
When the real money system is live, Blizzard will charge 1 USD per transaction for equipment that is successfully sold. A 15 percent charge will apply to successful commodity transactions, which includes crafting materials, gold, gems and "stackable" items. The money you make from a successful sale can then be kept in your Blizzard account and cannot be transferred back out. Alternatively, you could opt to cash out your item sale to a PayPal account, where the sale will be subject to another 15 percent fee and possible other charges.