http://www.techtip.org/why-honeycomb-android-3-0-is-not-open-source-yet/
Quote:
So, in order to prevent developers to fit Honeycomb into mobile phones, which would completely ruin the user experience, Google made Android 3.0 closed source.
But not to worry, the upcoming version dubbed ‘Ice Cream Sandwich’ will be open source. But Honeycomb will not be made open-source before that.