Been lurking for a bit but I've spent some time and money on this very issue so I figured I would chime in. I have a few that I go to relatively regularly. When solo with no messenger bag or backpack I use one of these...they are both relatively inexpensive. The "leather" sleeve doesn't provide as much protection but is really slick looking and works well in a business setting.
When in a backpack or messenger bag I use a Tom Bihn cache. These things are fantastic. You can put it on rails so you can pull it out of your bag or pack at the airport and keep it protected. It works slick as a sleeve as well. I find myself using this one the most as I'm almost always carrying a bag or backpack.