Morbidwrath on the Silvermoon forums pointed out a flaw in my educated guess for WoW Retention Rates. I missed an important factor relating The Burning Crusade sales directly to active subscribers: China does not yet have access to the TBC expansion.

The9 claims 3.5 million subscribers for their import of WoW in China and they've only recently reached an agreement with Blizzard to publish the TBC expansion.

Recalculating retention rates without China in the mix could mean Blizzard has lost 30% of the subscribers from their claimed base. Of course, this is still just observational theory. Only Blizzard knows their exact numbers and I doubt very much that they would publish them in full.