Sometime during Blizzard's WWI splash screen countdown, someone started a rumor that the ice-covered splash screen was a sign that Blizzard was going to allow Death Knights to be playable before the expansion launched. Somehow, people are still posting this idiocy, even after it turned out that the splash screen was for something completely different.
Let's completely ignore the logistical challenges involved in determining who has already pre-purchased an expansion that is not yet out.
And the fact that, if you're charging money and releasing some of the content, claiming that you haven't actually released the expansion early with only one of the major features is mere semantics.
And the possibility that players would be disappointed with the Death Knight (either because they dislike it, or because of its effect on other aspects of the game) and cancel their recurring subscriptions rather than await the remainder of the expansion's launch.
And the complete turmoil that adding a new class to the level 70 endgame at this late date would cause (including an ability that physically moves enemy targets to the DK, and possibly major changes to the Crushing Blows mechanic across the board in order to allow for tanking without a shield). Doubly so if we're also releasing the new talents in this hypothetical patch.
And the fact that, presented with the opportunity to allow low level BE's and Draenei when TBC was delayed, Blizzard did not do so.
Blizzard wants level 55 Death Knights on launch day so that some of the population will voluntarily go try a DK instead of swarming to Northrend.
Don't take my word for it, WoW's lead Producer told Tobold:
"When we actually launch Wrath of the Lich King you'll be faced with a choice of whether to create a Death Knight and start that process, or to take your main and level it up to level 80."
Can anyone tell me why, oh why, this sorry excuse for a rumor isn't dead yet?