Aesthetically, many beautiful objects have sharp corners - tables, windows, and computer screens, not to mention the universal reading tablet, a.k.a. the piece of paper. Our eyes haven't had a problem staying focused inside these rectangles.
The big draw for rounded corners is comfort. But we've been using sharp rectangular folders, binders, magazines, etc. for decades without complaint.
Apple used the rounded corner to great effect, and for now it seems like the only possible design choice. But some point, someone is going to produce an elegant matboard-thin computer tablet with "sharp" corners that don't cut your fingers, and suddenly the rounded corner will seem like a childish relic of the naughts.