Who makes compact backpacks specifically sized for Macbook Air 11/13"
I have a 13" MBA (bought in July 2011). I have the McKlein Montclare 15494 Small Netbook Laptop Case, and I love it! Here's a URL:

http://www.mckleinusa.com/15474S.html

Apologies though - not a backpack, more a messenger bag or briefcase sort of deal.

I own one in black. It exactly fits my 13" MBA, so obviously you'll have a little extra room if you've only got an 11". I like the amount of padding in the bag, and because it's on the thin side, it "encourages" me to keep my junk to a minimum.

FYI, I can slide my iPad 2 in next to my MBA, and that's about exactly how big the padded sleeve part is.