The current rules a device must meet to use the android trademark are at http://source.android.com/compatibility/4.0/android-4.0-cdd....

The short version from http://source.android.com/faqs.html#compatibility :

'We define an "Android compatible" device as one that can run any application written by third-party developers using the Android SDK and NDK. We use this as a filter to separate devices that can participate in the Android app ecosystem, and those that cannot. Devices that are properly compatible can seek approval to use the Android trademark. Devices that are not compatible are merely derived from the Android source code and may not use the Android trademark.'