The WoW Battlegroups Catacylsm and Frenzy are getting merged this week.

A full post is available on the Blizzard forums, a quote of the announcement is here:

We've previously announced our intent to merge the Cataclysm and Frenzy battlegroups in order to lower their arena and battleground queue times, and increase the amount of players available for the cross-realm PvP features. Due to the extensive maintenance this merge will require, we're currently anticipating a maximum of 24 hours of downtime for the realms in these battlegroups, which are listed below. We're currently scheduling this downtime to begin on June 19 at 3:00 AM PDT, and conclude at approximately 3:00 AM PDT on June 20.

This does lend tangible credence to the speculation that WoW's population is shrinking, or at least has plateaued.