So in the Blizzcon press interview with Robert Bridenbecker, Vice President of Online Technologies at Blizzard we got some new info.
We will not be region locked in D3 as we were in SC2. You can buy the game anywhere and play anywhere (with the exception for certain regions such as china), much like it worked back in Diablo 2.
That said you will be locked to your native region's RMAH.
What this means is that you for example if you registered in Britain then your default play region will be Europe and your default RMAH will be £GBP. You can change your play region to whatever region is available, for example North America if you want to play on NA servers. But you can only change the RMAH to other currencies in your registered region. For example you may sell and buy in €euro instead of £, or any other currency in the Europe region, but you cannot browse the NA $USD RMAH.
If you buy Diablo 3, you are just buying Diablo 3, you can play wherever you want. So you guys get to choose. Now what we will do is we will lock you down to a specific auction house, so for example if you are a European national you would have access to the European auction house. You will still be able to play in North America. You can participate in the gold auction house if you want, but you can only participate in the real money auction house of your native country [editor note: region]. [Another example] North American citizen, play anywhere, participate in the gold auction house anywhere, but the RMAH only in North America.
Regarding different region clients:
The download of the actual client will have no basis on where you can connect, all it's gonna dictate is what your language pack is... ... You wanna download the german client and play in Europe? you do that. You wanna download the german client and play in North America? you can do that too.
Q: So the functionality is closer to what Diablo 2 had where you could just choose server rather than Starcraft 2.
Full interview with more examples and some more bits of info: