The official Burning Crusade site has been updated with new concept art, new screenshots (primarily of the Draenei), and interesting lore information for both races. There's also a new gameplay trailer, so those of us who stayed home can see what everyone at E3 is seeing. (Well, almost.) There's a lot of other information out there, but Blizzard's site is still the official information, and it's the only place to find detailed lore.
[Image above is official concept art]