A 15'' MacBook Air is a terrible idea. What they need is to make a true successor the 12'' PowerBook from six years ago. Really, they should start with 12'' and go smaller. Basically, Apple needs a netbook.
Right now, I'm leaning towards a Sony Vaio P netbook. It's not cheap, but it's also the best netbook I've seen. That's what Apple needs to do. Build a netbook with a good resolution, strong construction and get it somewhere between $299 and $899. That would be a hot seller. After all, like Apple or not, I think most would agree their hardware is among the best in the industry. An Apple-quality netbook would be huge, I think.