Just my thoughts, but the one thing Apple will try to avoid at all costs is any kind of fiasco with the first batch of Ipads shipped.
Even if it's say 200k units, that is still a lot. Of course the goal isn't to just barely make the release date. There will be Ipads all ready to be shipped to your door the 3rd, which means they will be in the nearest warehouse before that.
As far as the Apple Stores go, I am sure they will arrive by the 2nd or earlier. I am sure that when Apple chose the April 3rd release date, they gave themselves a little cushion of time. As I said, the goal is not to just get the merchandise there in the nick of time, it's to have it all orderly and set to go with no glitches. I'm sure there will be a few, there always are, but I'm guessing it will go very smoothly, thus the change from End of March to early April.
What I do expect is that Apple is going to have a problem getting them made fast enough. After the release, and all the pre-orders are shipped, I wouldn't be surprised to see a big back log for those who waited. I think once these are out and more people see them, the demand is going to be huge. Just my opinion, could be wrong, often am.