Note: The photos don't show the whole laptop, just the screen, and it looks like the photos were taken in someone's SUV. There's also no actual measurement to show how large this screen is, so in all honesty there's no way to tell that this isn't the existing 11.6" screen.
Apple currently makes two Macbook Air models, one with a 13.3" screen and one with an 11.6" screen. The line was last refreshed last April, so it would make sense that a new model would be making the rounds right about this time (in anticipation of a spring launch).
But even so, I have trouble believing that the screen size is really 12". I just don't see the point, not when there is also a model with a 11.6" screen. (On the other hand, if Apple were going to go for a 12" and either a 14.4" or 15" model, that is different - but it's also pure wild speculation.)
Update: I've been taken to task by a reader for the lack if specific technical of Apple's products. I will repair this shortcoming, but in the meantime here is what Tim pointed out:
1. The Apple logo is brushed metal rather than a plastic cutout to allow the backlight through.
2. Unlike the Pro, the current Air has a gray plastic bezel around its surface. This switches to edge-to-edge glass.
3. (This is less obvious) but if you’ve seen the 11" on top of a 13", this screen appears to be about a half inch less wide horizontally than the 11" which points to a reduction in the bezel boundary around the screen.
Here's the screen unit in relation to the 13.3" Macbook
I would not buy one (I am wedded to Windows) but I would definitely want to go see it. There are Windows ultrabooks with 15" and larger screens, and I want to see Apple make all those thin light laptops look clunky in comparison.