The answer, it seems, is "nowhere." As far as I can tell, there are no user interface guidelines for Android. Microsoft provides design guidelines for Windows Mobile. Apple has pretty good iPhone Human Interface Guidelines, for web sites designed for the iPhone as well as for native iPhone applications. Even Symbian has some usability documentation. Google, it seems, has nothing.
Will this hurt Android?
In my opinion, from a UI standpoint, the results are rather sad. Don't get me wrong, the applications themselves are awesome. There are some great, great ideas, and I will definitely pick up an Android phone as soon as I can. However, the user interfaces are all over the place.
There is no consistency. Many applications invent their own iconography and nomenclature for things. Most applications have custom buttons, windows, color schemes and widgets. Applications use different typefaces and font sizes, sometimes inconsistent within the same screen. The icon style seems to be undefined; some use flat icons, some have realistic pseudo-3D icons, some have even other icon styles; they all look different from each other.
Even worse, many applications have small, tightly placed buttons. These interfaces will be useless without a stylus; it seems these apps were designed to run inside the Android emulator, where they can be controlled using a mouse.
Frankly, while most of the application ideas themselves are awesome, the inconsistent (and, in a few cases, just poor) UI design is disheartening. Hopefully, these UI issues will be fixed before users get to try these applications.
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