Every new Android SDK Version means that someone has to collect the sources and bundle them. Thanks to the supporter of the missing source jar issue a new source jar is provided. Also please star the issue to flag that issue as important!

Install instructions:

To use it in Eclipse, create a directory sources/ inside your
/platforms/android-1.6/ directory, and unzip the archive there. Start
Eclipse and check that you can see the source, for example ask for the definition of
the Gesture class (new in 1.6), you should see the source code, not the decompiled
byte code.

We will mirror the sources to, so feel free to download and use it!
Android SDK 1.6 sources