My order says ships by as well - if you scrutinise the site the iPads themselves are deliver by 28th and the accessories are ships by 28th.
Maybe if you order accessories you get a ship by 28th and just the iPad deliver by 28th on the order.
Hopefully all of it means deliver by 28th, although maybe this is a sneaky trick to help Apple with the volume of orders, anyone with accessories will get them a couple of days later?
Must depend on accessories. I ordered a dock and I've got "delivered by". People reporting "shipped by" seem to have ordered the case.