Maybe not. During a Q&A session at GDC Paris 08, Blizzard VP Rob Pardo was asked to comment on "the new game." He replied saying, "So you want me to announce the game before the announcement?" That was a classic press conference or Q&A blunder; you're not supposed to accept the premise of an unwanted or hostile question!
If there was something to his choice of words (there might not have been, but hey, it's fun to speculate), then Wrath of the Lich King theory might be -- pardon the expression -- on thin ice. Would you call an expansion a "game" by itself? And hasn't WotLK already been announced? All hope would not be lost for MMO fans, though. Diablo 3 could be an MMO. The game could be World of Starcraft, or it could be an MMO in a totally new franchise. Remember, Blizzard has been hiring staff to work on an unnamed "Next-Gen MMO" for months.
Despite Pardo's comment, we're not going to write WotLK off yet. The rune-adorned ice in the image looks identical to that found in existing WotLK artwork. Just compare the splash page to the ice beneath the sword at the expansion's website.
Stay frosty. We'll probably get answers at Blizzard's worldwide invitational this weekend.