Recently the Blizzard website main page switched over to this new splashpage. It's intriguing. Since we know that something is indeed coming, this only confirms that there will actually be an announcement of some significance on May 19th.
Looking at the graphic I noticed a few things. First, The Burning Crusade is not listed separately. This leads me to believe that the announcement isn't simply an expansion, but an entirely new game. Oh, the curiosity burns within me. What will it be? Also, I see that the games Warcraft: Orcs and Humans and Warcraft II: Tides of Darkness are highlighted, but not Warcraft III or World of Warcraft. Perhaps this is a game that is centered around the first two games in the Warcraft canon. Is it World of Starcraft as the leaks have said, or is it Diablo III? Is it something else? Why can't it be the 19th already!
[Updated 5/13/2007 7 pm EST]
In searching through forum posts about this topic, I came across a fabulous summary by Colrum of Shadowsong who references some interesting tidits on Blizzplanet. They are citing recent events, such as the hiring of RTS guru Dustin Browder in 2004 and the May 2007 release of a new Starcraft novel, as evidence that we may be seeing a new Starcraft RTS rather than an MMO. Take this how you will, but it sounds very plausible to me.
Also of note, several people have said that this graphic is a countdown page. This is substantiated by the HTML source code which labels the image as "day1.jpg." Looks like we will have a countdown similar to the Worldwide Invitational website. It only remains to be seen what that question mark reveals on the 19th.