Rollie has posted his June concurrency numbers for World of Warcraft over at WarcraftRealms.com. The numbers indicate that the drop in activity in May has ended with a turnaround in June. June player activity is up sharply and WoW is still maintaining a higher player activity level than prior to The Burning Crusade's launch. All positive signs that TBC is being played more often and by more players than the original game.
The numbers are based off data collected via the CensusPlus UI mod and thousands of users across nearly every WoW server. Rollie has been dealing with WoW population numbers since the game launched, so his data and analysis is fairly useful.
Without confirmation directly from Blizzard, the noted trend is speculation. However, given a couple more months of data collection and analysis we should be getting a relatively reliable pattern. Rollie already has two years worth of back-logged data to work with.
It will be interesting to see who picks this month's information up and compares it to the May numbers. A lot of naysayers were jumping head first onto the "TBC killed WoW" bandwagon, but another month has come and the numbers have evidenced against their claims.