Why would Apple build a netbook when they're "just a fad," as you say? Not to mention that Steve doesn't see the appeal of netbooks, anyway.
Not to mention that the primary appeal of the netbook market is the price. If Apple builds a netbook that starts $500 or more, I doubt it will be all that successful. The Sony Vaio P, for example, is very tempting, but starts at $900, which makes it more of an ultra-portable than a true netbook.
First off, I said Apple would build something in the same size factor, not the same form factor. What I expect is something more along the lines of a note-pad-sized tablet; perhaps equivalent to doubling all the dimensions of the current iPod touch to make a unit about the size of the current netbooks without having to put it down to use it. True, it will likely be more expensive than the current netbooks, but it will have them totally outclassed in usability.