First, if you look closely at the pictures, there is virtually no way that this was pulled together as a Photoshop mockup -- it would have taken much less time to build one from scratch. Therefore, the thing is likely real.
Next, the keyboard could be one of the following:
a. Cosmetic Mock-up: A non-functioning prototype used to establish help validate the basic design premise and, specifically, the look and feel of the thing. It is one thing to work up a conceptual design on paper or in a CAD system. It's another thing entirely to actually be able to feel the concept in your hand. Manufacturing of mock-ups is very easy using CAD/CAM (a CNC mill would be able to crank out the aluminum substrate in a matter of minutes), along with the application of "dummy keys" scavenged from the Macbook manufacturing process. Note: there may be many different versions of a keyboard, each different (straight vs. ergo, blue vs. green, etc), each being made to test the design concepts.
b. Functional prototype: A functional prototype may or may not have the cosmetics of a final prototype or the final candidate release of the product, but it would help to establish the mechanical attributes of the keyboard. In this case, the key placements, angle of keyboard, location of the USB port, the "feel and response" of the keyboard, etc.
c. Candidate release or prototype: There are many names for the prototype in this stage of development, but the concept is the same: this is the near final manifestation of the keyboard, ready for manufacturing. This unit might require some final tweaking before being submitted to manufacturing, but it wouldn't be much. For instance, I noticed a huge argument over the Apple logo on the Option key... This would be one of the things that might be getting worked out in the final phase of engineering, and has a great deal to do with the design team's efforts to nail down the aesthetics. This stage also helps the manufacturer ensure that their tooling and manufacturing processes are nailed down before they start manufacturing millions of these things...
d. Final manufacturing release: This is the product that you will find on the counter of your friendly Apple Store, once the thing is released. It doesn't imply that this will be the last iteration -- there may be additional changes to the keyboard, and those will be tracked under separate release branches.
Given the labeling on the back of the keyboard, I'm guessing that this unit is a late-stage functional prototype, or an early-stage candidate release. The keyboard looks very well defined and refined, and the fact that it states that it is not a cosmetic unit (a non-functional unit) means that that stage is now behind them. Apple and their manufacturing contractor probably constructed hundreds of these prototypes to ensure that they have all aspects of the keyboard buttoned up. This may not be the absolute final manifestation of the keyboard, but I'd be willing to bet that it isn't that far off from what we'll see in a week or two.
As far as the design, I think it is a stunning keyboard, and I think that Ives & Co. have done a superlative job in the design. They are known for their minimalist aesthetic (of which I am a huge fan), and I look forward to having one on my desk. As far as ergonomics are concerned, after 25 years of pounding away on non-ergo keyboards, my wrists are appropriately seasoned for a standard keyboard construct. One of the posters mentioned that this keyboard isn't at the right angle, and that flat is not good. With hundreds of millions of laptops in circulation, each with a flat keyboard, I'm not sure I get the point that the poster was making... If nothing, by consolidating the breadth of components down to a minimum across all platforms, Apple will continue to improve their margins and manufacturing process; not a bad thing with ever increasing competition...