The brilliance of what Apple has done with their mobile products is hard to overstate. They put all this effort into designing mobile chips and a mobile OS for the iPhone and iPod Touch. They took the fruits of those efforts and stuck them into a tablet format, and bam, you've got an iPad. They took them and stuck them into a set-top box, and bam, you've got the Apple TV 2G.
They get to spread the development costs of their silicon and OS over these four product lines. Talk about efficient reuse of your development investment. I own three separate devices powered by an A4 CPU and iOS operating system.