For the last few years, I've been using the Sons of the Storm website and various Blizzard artists' DeviantArt galleries to get my Blizzard concept art fix. As of this evening, it seems Blizzard is taking a step toward consolidating all of this artwork into one public place.
Early days is right! There's artwork in the gallery that predates StarCraft and the early Warcraft games. Go check out Blizzard's concept art gallery, it will be time well spent.