A source close to the E3 show floor has helped confirm our suspicions that the new Alliance race, as featured in World of Warcraft: The Burning Crusade expansion, will be revealed at the show. The information we got was vague, but encouraging:
Looking at a video on the show floor, the new Alliance race is ugly with grayish skin, and glowing eyes similar to the Forsaken. Its chin has two things like tentacles hanging down, one on either side, and it's humanoid and human-size. There's also a huge picture of Antonidas nearby.
This seems to rule out a lot of contenders, though there isn't an obvious race that springs to mind from this loose description. It could be the Eredar, the Broken or, at a stretch, Forgotten Ones -- possibly even oddly rendered Draenei. As for what Antonidas is doing there, we'll find out more on Wednesday when E3 opens.
[Update 1 - added The Broken as a possibility, thanks RaptorJedi]
[Update 2 - Our informer writes, Having researched this further I am certain that the race is the Broken, the Draenei faction that remain in Outland.]