They got a design patent for the iPad's design. Not for a rounded rectangle. Comp the first comment on arstechnica that jccc posted here.
Jonathan Ive said it best: great design looks obvious in hindsight.
edit: Gotta love the downvotes. Must have hit a soft spot. :)
I love how people think they get downvotes only because they hit some hypothetical soft spot. :)
What's important is that design patents are really really narrow in scope. A would-be imitator would have to do their best to copy them exactly (like Samsung did with their iPhone copycats) in order to infringe. So no, it would not be enough for a company to have a rounded rectangle device to get in trouble.