Instapaper was an idea, and Marco is pretty firmly in the camp that you can't steal "ideas" (Though, you can do feature-by-feature clones, which another of his competitors did in a really annoying fashion. )

Yes, and he has discussed his fear on the launch of Read it Later, but the non effect (it was it a slight bump?) on his sales. To me however, that is a courteous approach. Apple saw and copied with a better implementation that 3rd parties couldn't copy.