Diablo 3 Realms
In Diablo 3 there will be different realms divided by Region. For example, North America would have their own realm, Europe would have their own realm and Asia Pacific would have their own realm. Your Realm will be chosen by the Region in which you live (Eg. U.S., Europe).
Players in one Realm will be able to chat and send messages to other players in another Realm. However, each individual account can only play Diablo 3 in one Realm (Also known as "Realm Lock"). When asked about switching accounts between Realms, this was the following response from a member of the Diablo 3 Development Team, "No plans at this time. I would count it among ‘pretty unlikely’ due to the real currency ramifications."
Since cross-realm play will not be available, exchanging or trading with different currencies will not be possible either. If you are in the U.S. Region you will only be allowed to use U.S. Dollars in the Real Money Auction House. This also proves true for players every other Realm/Region, that they can only buy and sell with their respective currency.
Though nothing is final until release, it does not seem likely that Blizzard will change their minds on this.