This is an intriguing question, David Janes asks in an insightful article (title of this post taken from there).
He then shows a smartly stitched graphic:
(small pic here - see the impressive original)
Showing a "translation table" between Google Maps, Yahoo! and Virtual Earth, David develops sort of a generic, 'universal' zoom notation. Level 19 is put equal to 'ground' (Y counting reversely). The smart endnote reads:
A truly universal zoom level would just use altitude; dealing with a simple integer is easier and less complication prone though.
