I am also considering the Crumpler bags. There are a few that look like they might work well with the MacBook.
The Hee-Goer says that it fits the 12" i/powerbook and the 13" MacBook. I wonder if it would be a very snug fit, hopefully not too snug.
The Moderate Embarrassment says it will fit the 12 inchers, but it does not say that it will fit the 13" MacBook. I wonder if it would fit the MacBook, seeing as the Hee-Goer was originally for the 12 inchers? I guess there is no way to know.
The Dreadfull Embarrassment can fit up to a 17" 'book, but I am considering it because it would probably have a lot of room for books. It should be safe enough without a sleeve, but paired with a sleeve it should provide adequate protection for any abuse you throw at it.