To lock my orientation to portrait, I use:
activity.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
I'm unsure what flag tells the activity to go back to relying on the device orientation. I imagine it is one of these:
SCREEN_ORIENTATION_FULL_SENSOR
SCREEN_ORIENTATION_SENSOR
SCREEN_ORIENTATION_UNSPECIFIED
SCREEN_ORIENTATION_USER
On another note, why isn't the android documentation open source? The documentation is completely lacking. Very few of the functions and flags have useful descriptions.
Answer
Per http://developer.android.com/reference/android/R.attr.html#screenOrientation (screenOrientation being what those values are linked to if you dig through the documentation), SCREEN_ORIENTATION_SENSOR or SCREEN_ORIENTATION_FULL_SENSOR will do it, depending on how much flexibility you want -- however, I suspect what you really want is to go back to the default setting, which is SCREEN_ORIENTATION_UNSPECIFIED so that it goes back to the system defaults, including any the user set.