Oh, I don't know.
Call me suspicious if you like, but if they had kept, as almost everyone thought, to their expected "One year since the last launch" they would of had another 4 to 5 weeks to ramp up production and build up stocks around the world so they could of launched with good stocks in shops and on-line.
Instead they launch 4 to 5 weeks early and just don't have anything like enough to even have much stock in stops, let along on-line.
Surly Apple don't plan THIS Badly do they?
I imagine you are "suspicious" based on your belief that the Xoom presented an existential threat to iPad and that Apple had to announce early to prevent iPad sized lines from forming outside Verizon and big brick and mortar stores to buy the latest and greatest mobile phone OEM/Google creation.
We have to wait to see what the official sales numbers look like, but it could be a situation where a month would not make any substantive difference. iPhone 4 has basically been in a perpetual state of constrained production, and iPad 2 could be the exact same thing.
I think that it's safe to say that they are manufacturing as many iPads as possible and if they were waiting to be able to maintain shipping within a week on all models worldwide from the launch date, there wouldn't be an iPad 2 until iPad 3.