And how much a 15" MBA would cost? Ultimate 13" MBA today already cost around $1600, which perform half good if you configure your own MBP with the same price (you can add RAM, SSD, processor) etc. And you paid the price for being a bit thinner and lighter.
Now if 15" MBA going to be rolled out, not only it beats the purpose of Air being ultra portable, but also cost could be insane, maybe the top end would reach $2000 .. for a DVD-less netbook, non upgradeable RAM and only 2 usb ports.
Being that expensive, pick up base iMac for $1100-ish and basic MBA for $900-ish should be a much better option. You can get faster processor on your office/home, while maintain excellent portability with 11" base model MBA. And with the new model, you can connect them both via Thunderbolt cable. Done
I even wonder why Apple would create 13" MBA, because 11" fits the bag really well. And that's the point of having Air, having a full featured Mac computer that fits on your bag without making you remember you brought it