The recent netbook rumor said that some $99 3G netbook would be released this summer, and it won't run Windows. For some reason, it was posted here. It seems quite obvious that it's probably a Linux netbook that will be subsidized.
If Apple ever does anything like this, it'll probably be more of a larger touchscreen device with a keyboard of some sort. It would take Apple actually not trying to make THE best product on the planet, but it could be done for maybe $800. The problem is netbooks are around $250 at the low end, and that's who buys most netbooks. People who want more expensive computers will buy a MacBook or a MBA. Thus, Apple would have to make a product that stands out and would be able to sell at a more premium price, which a touchscreen device would.