I've been trying to find out the same thing. I've called Apple's 1-800 number and they said that they get the same info as everyone else from the website (which I doubt). I've also contacted MacMall since I needed to buy something else from there and they said they didn't know either.
If I were to guess, I'd say that the 21" preorder would be the first week of November and the 27" would be the first week of December. The only thing that I'm afraid of is that the 27" will fall into shipping towards January/February timeframe.