It doesn't make any sense to "hold back" hardware for an OS update, when there is no stock in the stores and customer demand. Can you imagine if you went to a car dealership to buy a car, and 10 others were waiting to buy a car and they said "sorry, we're out of cars for the next 3 weeks because the new models are going to have a new feature" ... that's absurd, and that's not how marketing has ever or ever will worked.
It actually does make quite a bit of sense - it means more Lion adoption from the beginning, instead of having to rely on customers to go upgrade to it after getting new computers (the majority probably wouldn't for a while). Apple has done this before, and I have no doubt that's exactly what they're doing now. Aside from that, Apple stores still have them in stock today, or at least the two stores within 5 miles of me do...
Oh and there's no way they are waiting to release them until the fall.