I think they didn't want to announce a " ORDER THIS FRIDAY AT MIDNIGHT!" because they figured their stock can't handle the demand. They don't want people angry that it sold out in a minute and had no real shot at getting one at all. I know I feel a bit annoyed when that happens. Sure, it makes for a great headline, but it's still not ideal.
Also, if the low-yield rate of the iPad is true, this could be seen more of a soft launch, gearing up for black friday week sales. It makes the sales seem steady.
If anything, the mini could look TOO HOT, if it was selling out like crazy while the Air is still within 5-10 days shipping.
I am a bit surprised it isn't a sellout into December yet - however, even though it is already 1PM (PST), I bet there are tons of people who really want one and still don't know it is out!