One of the accessories that will be available for the iPad at its launch is a dock with a full-sized keyboard. On the top row of the keyboard, where the Escape, F1 - F12, and Eject keys are on a standard Mac keyboard, are 14 keys geared toward the iPad's interface. In place of Escape there's a dedicated Home key, then a Search key, keys for increasing and decreasing screen brightness, a key to activate the photo portrait screensaver-like function, a key to invoke the iPad's on-screen keyboard, a blank key, three keys for controlling media playback, and -- hey, wait a minute. A blank key?
MacRumors forum member macduke noticed this completely blank and apparently functionless key, and 9to5Mac picked up on his speculation concerning the mystery. 9to5Mac noted that it's "...silly, even for Apple" to have a key on the keyboard that does nothing and goes nowhere, and they theorize it could have something to do with a Dashboard-like interface on the iPad. Notably, Apple-built applications like Stocks, Weather, Voice Memos, Clock, and Calculator were all absent from the iPads people played around with following its unveiling, which has led to a lot of speculation that such simple, small apps could be bundled into a Dashboard layer on the iPad. If that's so, this mystery key could easily have a Dashboard icon on it when it finally does ship.
It's worth mentioning that the F5 and F6 keys on Apple's aluminum keyboards also currently have no dedicated functions -- those keys are used on the MacBook line to increase and decrease key backlighting -- but the iPad keyboard dock is also missing a function key, making it unlikely that the top row of keys will be usable for anything other than the functions indicated by their on-key icons. Although it's conceivable Apple left this key blank in order to allow users to define their own shortcut for the key, it seems far more likely that Apple is working on some special functionality for that key which it wasn't ready to show off in January. Given the absence of several of Apple's own apps from the iPad, Dashboard seems like a very good fit.