As of January 2012, developer.android has a rather decent design section at http://developer.android.com/design/index.html. This is a good place to start.
While developer focused, this is a great list of resources for Android OS work.
Android Design from developer.android
Android User Interface Guidelines also from developer.android
Listing of Android patterns, real ones, with problems and solutions as drawings, from UNITiD in Amsterdam.
Android Drawables Graphical list by OS version of every "drawable" (icon, control, etc.) in Android. Indispensable tool. I live by it.
Switches just an example of the Building Blocks pages on the Android Design site. Very useful overview of the widgets available.
Stencils & Templates:
The Photoshop PSDs Google forgot Layered documents from Joseph Richter. (Photoshop of course)
Android GUI PSD (High Density) from Teehan-Lax.
Library of Android UI Elements from Smashing Magazine. Photoshop library of Android (default) elements only. (Photoshop)
Android 2.2 Elements based on the Smashing Magazine work. (Photoshop)
http://www.webdesignshock.com/freebies/free-photoshop-android-interface-gui/ which has a lot of UI widgets, and includes the typography needed to make it all work.(Photoshop)
Icon Design Guidelines, Android 2.0 from Developer.Android.com. Includes ways to download their template pack and other useful stuff. Quite detailed for the graphic designer, for a change. (Photoshop)
14 Android GUI Widgets by which they seem to mean default desktop widgets (e.g. clock), vs. UI widgets like scrollbars. (Photoshop)
HTC HD2 PSD Vector150 layers, 60 groups, almost all vector. (Photoshop)
HTC G2 370 layers, almost all vector, (Photoshop)
Motorola Droid 2350 layers, almost all vector (Photoshop)
Samsung Galaxy S Device not specified. 127 layers. (Photoshop)
Samsung Galaxy Tab P1000 110 layers. (Photoshop)
Tests & Defaults:
API Demos Go get this. Really. It has lots of nerdy stuff to check for capabilities and so on, but is a great way to for sure know what default capabilities and UI are. Go into, say, Preferences then pick Default values and you can see the default behaviors of Android Preferences. Useful.