That turned out to refer to automated testing harnesses as opposed to live users (see: http://developer.android.com/tools/help/monkey.html). Unfortunately, http://developer.android.com/tools/help/goat.html 404s.
The Android API is known for lots of little hidden references and Google jokes. The SensorManager for example has a bunch (i.e., GRAVITY_DEATH_STAR_I and SENSOR_TRICORDER: http://developer.android.com/reference/android/hardware/Sens...).
According to the Chrome team, the "Goats Teleported" reference in Chrome is just a random number: http://www.quora.com/Google-Chrome/How-is-the-number-for-Goa...
So, it appears that isUserAGoat is most likely just an engineering joke that shipped.
I believe that's a Google intranet page, and so this story is left hanging for the non-Googlers among us.
(IIRC, the original Mac also had an area of memory called "MickeyBytes" that stored the current mouse acceleration settings)
If you're interested, drop me a line at 1.800.GOAT.API and I will license my patent for your app.
 Cf. http://stackoverflow.com/questions/13248493/detect-ipad-mini...
 Also relevant: http://xkcd.com/378/
(link found on some other page, can't remember where)