Actually, I'm pretty sure, that HP just doesn't know it for themselves.
They only can fix a date, when webOS 3.0 (including the underlying development platform) is ready.
Until we hear about that, it won't be possible to flash the devices with the final OS and therefore they can't sell them.
I am pretty sure, the devices are ready hardware-side, but still lacking the software.
And remember...Leo said, he wants to have a "perfect" product, so I don't think, they will release it until it is truly finished...
especially as Apple won't release anything until fall, there is absolutely no need to rush anything out.
They better take some more weeks to make it "perfect"!