That is correct, all arenas are now on one server, the original MA server from savvis that was moved to AT&T.
All pevious arenas were switched to the second server at savvis until the AT&T one was up. So the CT, DA, TA, SEA server at savis was also handling the MA.
Once the first AT&T server was up it replaced the last savvis server and took the whole load or all arneas. The last part of the move is getting the remaining server from savvis to AT&T and when both are installed @AT&T I'm sure HTC will distribute the arenas in a similar fashion as before.