The Flat Aluminum Keyboard is Apple's most recent USB keyboard, shipped with all their Macintosh desktop computers including the iMac. It is also sold separately, and as a standard USB HID device, can be used with non-Apple hardware such as PCs or Sun workstations.
In essence, the Flat Aluminum Keyboard is exactly what it says on the tin: it's thin, it's flat, and it's made of aluminum (with plastic keys). It's essentially the scissor-action keyboard of the newer MacBook series, transplanted into a housing to be used with a desktop machine, though it does include a full numeric keypad. Indeed, it's actually a fairly typical Apple 104-key layout. It's also one of the smallest and lightest full-size keyboards I've ever seen, which is somewhat surprising considering its construction. (While aluminum is very light for a metal, it's denser than most plastics, which are what most keyboards are made of.)
Unfortunately, it sucks. Now, your mileage may vary on this - indeed, I know a few people who swear by the FAK - but most people I know of swear at it, instead. It feels really odd to type on, with a longer key travel and a spongier feel than most laptop keyboards, but much less travel than most desktop keyboards. Also, it's so thin that it feels rather flimsy. This is probably less of a factor for those with smaller hands, but it really looks and feels delicate. Unlike most PC keyboards, there's a distinct sense that if you dropped the FAK on the floor, it would probably come out markedly the worse for it. In particular, the light-gauge aluminum is rather dent-prone. The overall feel is rather the opposite of the durable, solidly-built MacBooks that it is patterned after. In the actual event, it is rather more durable than it feels, but feel counts for a lot.
Typing on it gets weird quickly, for much the same reason that typing on a laptop keyboard gets weird. The close spacing and short key travel is rather unforgiving of fat-fingering. All of the keys are perfectly parallel to the keyboard face, which, combined with the slight cant that the keyboard sits at due to its integrated USB hub, means that your fingers won't sit on the keys quite the way they do on most desktop keyboards - or most laptop ones, for that matter, since those are usually flat.
Other nitpicks are the fact that the cable is short and non-detachable (annoying if it breaks), though Apple thoughtfully includes an extension. This seems uncharacteristically klugey for Apple, though. There's also a bug that makes it possible to slip a keylogger into the keyboard's firmware. This bug is possibly showstopping for high-security environments, but because you need to have root/Administrator access on the computer to which the keyboard is attached in order to do this, it's unlikely to be a problem for ordinary users.
On the plus side, it's pretty, it's easy to keep clean and it has a very compact footprint for a full 104-key keyboard. It also requires rather little force to type on. This is both a positive and a negative, it all depends on what you're used to. All in all, I think the FAK would be fantastic for children, and for people who really like the feel of a laptop keyboard. It seems like a very odd choice, though, to ship what is on the face of it such a niche item as the default keyboard with all new Macs, though.