A lot of people are saying 60 days or 90 days which is actually the ship date. Nothing has been stated by Apple as to when it will be available for preorder on their website. Although if you'll notice on their video advertisement and on the specifications page (at the very bottom), there is mention of the iPad pending FCC approval. Since it's illegal to sell a product that requires FCC approval I would assume that this is the only reason it is not available for preorder right now. I think it's safe to say once approval has been granted we will be able to preorder it.
For those of us who remember waiting for the first gen of the iPhone it was sort of the same story, along with the FCC notice. I'm sure the snowing of the federal government the last 2 days isn't going to help the process