Thanks for the suggestions. I'm trying out the incipio sleeve, so far. Not the leather one, so it cost me $20, total. It's technically made to go in the bag, though I'm still wondering if that dip in the top leaves too much unprotected.
Before I swore by the Incase sleeve, I had an open flap style sleeve for my rev C MBA, but there were scratches on the exposed part of the laptop after a year of traveling. Granted I would put the sleeve in the same compartment as folders, reading, magazines, etc but that's why I went with the Incase, as it is a total wrap.
On eBay, it should run you ~$25.