There are renewed rumors about Apple working on a small ultra portable notebook. Despite Steve Jobs' comment that he did not know a way of making a computer for $800 that was not a hunk of junk these rumors have persisted.
The latest rumors seem to have some traction though. According to China Times three players in system manufacture have received direct orders from Apple. Dynapack, Wintek, and Foxconn.
The new book should be about 9.7-inches (just to be different) will be a touch screen (ok Multi-Touch) and should be in the $800 range. It is interesting that Foxconn will be making the new book as Quanta has taken over much of the manufacturing for MacBooks.
Of course this is all rumor and might not be a netbook, ultrathin or anything at all resembling one. Apple has a history of doing their own thing and I cannot see them trying to enter and really compete in the netbook market. This is especially considering the "cheap" label that netbooks have in the market.