Tyren from the World of Warcraft forums has recently posted an announcement which clarifies that: YES, those "Are you ready for The Burning Crusade?" email that you have been receiving are indeed from Blizzard, and are therefore legit.

According to him, the emails were targeted towards inactive or trial account players, reminding them that if they want to play The Burning Crusade, that they better download the necessary clients and patches early to avoid the rush when TBC launches in a few weeks' time.

In any case, the header of the official email reads:

While you've been away, something monumental has been happening in the World of Warcraft. Beyond the Dark Portal, the sinister agents of the Burning Legion have been waging their demonic crusade to consume the magic of the universe and lay waste to all in their path. On January 16th, their Burning Crusade will come to Azeroth.

Blizzard Entertainment proudly invites you to return to the World of Warcraft on January 16th and journey beyond the Dark Portal, where an infinity of new experiences await you. Given the high volume of returning subscribers we expect when The Burning Crusade expansion goes live, if you are planning a return to Azeroth, we recommend reactivating your account as soon as possible in order to avoid the expected rush of launch-day activation.