This rumor has been around for quite some time and Steve Jobs himself said they have looked into it, but not interested at the time. Apparently Apple has realized that the market demands more affordable notebooks from every manufacturer. The company Wintek has supposedly stated they are making the touch panels for a new netbook which would be released in the 3rd quarter of 2009. The rumor is substantial as it comes from Digitimes, who has been accurate in the past.
Apple must know that all of their Mac followers would purchase a netbook and many other would pick one up as well. It is hard to say what the sales figures of such a netbook would be, but you can bet it will be an instant hit if the price is right.