Dustin Browder (Cavez) said the Zerg race is not ready to be unveiled. His last remark makes you wonder if the Zerg announcement would be made by the time Wrath of the Lich King beta is announced. Just speculation.
Dustin Browder: We will give you Zerg stuff just as soon as we can. We aren’t holding it on purpose, it’s just not ready yet. We had a lot more time to work on the Protoss and the Terrans before the announcement that you guys didn’t see (because the game wasn’t announced yet). (Source)
I don’t have an ETA but we won’t be waiting for the next Blizzcon. We are quite happy to put stuff out on the web for you guys to check out. It doesn’t HAVE to be a giant event to show a race, though when an event happens to match up with a race announcement that is a lot of fun.=)