The MacBook Air is so tiny that you'll be tempted to just toss it into a portfolio, briefcase, backpack or whatever you happen to be carrying at the time–but I have to caution against this. With miniaturization come inevitable tradeoffs in durability and ruggedness. The MBA should never be mistaken for a Panasonic ToughBook and will not be able to take the abuse that even a normal five pound notebook can withstand.
That being said, a quality padded sleeve is a requirement for the MacBook Air. Like some of you, I got swept up in the hyperbole surrounding the MBA announcement in San Francisco last month and couldn't resist plunking down my US$30 for the AirMail sleeve from Manila Mac. It's that padded sleeve that looks just like the manila folder that uncle Steve's pulled the MBA from during his keynote address. The problem is the it's delayed and I can't wait around for them to ship.The best sleeve case for the MBA, hands down, is the MacBook Air SleeveCase from WaterField Designs. The size 13-2 SleeveCase is designed specifically for the MacBook Air and fits it like a glove. They brought sample cases to the first day of Macworld Expo anticipating a new subnotebook and hastily put them into production after they confirmed the fit. The Waterfield 13-2 SleeveCase began shipping on 25 January 2008. The basic 13-2 SleeveCase retails for US$39 with a open top, side pocket and velcro closure. This is my preferred model for using while inside another bag. If you're looking for your sleeve to be the only case for the MBA (as many people will) I recommend that you upgrade to a flap (US$15) and if you want to carry it on your shoulder you can add D-rings and a strap (US$12-18). If you want a little extra room to carry an AC adapter and some dongles go for Waterfield's piggyback case (US$22). I've been using and recommending Waterfield sleeve cases for years because they're super-protective and built in the United States from high-grade neoprene and ballistic nylon. Plus they look pretty damned good too.
What's your sleeve/case strategy for the MacBook Air? Are you just going to carry it in a manila envelope?