There is no requirement in any RAID system that I'm aware of that all of the drives be identical. You will only get as much space as if they were all the same size as the smallest one, though.

What the Drobo does isn't exactly groundbreaking. It's been doable for years, if not decades, given a logical (extent-based) volume manager and a filesystem that supports online resizing. All they've really done is wrap some scripts around it.