Apple sells a high-end product; anyone who might be tempted away by a netbook would likely have already been tempted away by a lower-cost PC. It's the same reason that inexpensive Japanese cars don't cannibalize BMW sales.

Still, it seems weird to put those into different categories. There could be a PC maker creating expensive PCs that don't compete with netbooks. In fact, there probably are (for example creating high end PCs for architecture and film editing). That PC maker would not be cannibalized by netbook sales.

Maybe if it is Microsoft vs Apple they are talking, but MS also sells stuff for netbooks.

Also, if there were no netbooks and no PCs, there probably would be more Apple sales. So Apple loses sales to PCs and therefore also to netbooks.

Of course if you are really smart you build the same car and sell it as an Audi (to mac owners), VW (to laptop buyers), a Skoda (to desktop buyers) and a Seat (to netbook buyers).

Whereas if you are GM you build 10 different cars with different badges from different parts that all compete with each other in the same market.

Also explaining why the Japanese car makers have built more expensive product; the two markets overlap but aren't the same.

Japanese cars absolutely eat into BMW's sales, or have you not noticed the army of hybrids gliding around?