The new alliance race has finally been revealed in the New York Times. It's the Draenei - a race that's long been speculated as the most likely choice. Other than screenshots, there's not much solid news yet on the play abilities of the Draenei, so stay tuned for more news as it comes in.
Update: There's also a New York Times interview with Jeff Kaplan (AKA Tigole) with additional details, including the possible classes: warrior, priest, paladin, mage, and hunter.
[Thanks, Stephen, Shawn, and Geoffrey for all sending this in!]