Doh.. it figures ...
I *just* bought a 13" Air for my wife for Mothers' Day, and now the new model is on the way....
Really though, I thought the current Air was perfect for her needs when I got it, and don't see why she'd make any use of the improvements in the upcoming model -- so it'll do. The SSD makes these machines so responsive and makes boot time so short already, the upgraded CPU is going to make a FAR less noticeable speed improvement.
I tell you what though.... I was torn on the 11" or 13" question at first, but after about 30 minutes of going back and forth between them, I determined the 13" made more sense, all the way around. You're not paying that much more money to go with it, but you get a screen resolution equivalent to previous generations of the 15" Macbook Pro on it. (1440x900) That's a truly useful number of pixels, and still looks perfectly sharp and readable on their 13" display.
Meanwhile, the 11" feels too much like the cheap "netbooks" to me. You compromised screen real-estate and got a slightly slower CPU, and for what? To shave 2 lousy inches off the dimensions? What's that really going to do for you, besides an initial "Look how small my OS X portable is!" bragging rights thing? Either one is super thin and light-weight and fits pretty much anywhere, and don't forget -- if you need to take the AC power adapter along with you? It's the same size for either model, and means you need a considerably larger carrying bag of some sort so it can be packed away safely with the notebook. So that, alone, can negate any savings you thought you got in size by going with the 11".
WOW! If this happens, AND backlit KB, I may need to consider selling my 13"MBP for one of these! Can't say if I'd go 11" or 13", but having used an MBA for a bit, they're sweet! Especially the SSD's...wow.