The docs state:

"myMap = native.newMapView( left, top, width, height )

Renders a MapView within the specified boundaries and returns a Corona display object. This object can be moved or rotated like any other display object."

However they do not behave like regular display objects as they cannot be added to other display objects and other display objects cannot be added to the mapview object.

This causes two problems:

1. A map view cannot be used within a tab view
2. Buttons cannot be loaded onto a mapview

..and you can't place anything on top of a mapview :(
I'd love to use a mask or some transparent png on a mapview!