Google's latest "Platform versions" breakdown indicates that Android 4.0-4.0.2 are installed on 0.3% of the Android user base. These are the OS versions loaded on the Galaxy Nexus (4.0.3 is released as open source and has partially rolled out to the Nexus S, but is not officially available on the Galaxy Nexus yet).
Google recently announced 700,000 Android activations a day, and if I extrapolate back from the last announced figure, I estimate the total Android userbase to be roughly 250 million, meaning total GNexus sales are 750K. Taking a WAG, maybe 500K of those are the Verizon version.
Incidentally, the same logic tells us that the total Honeycomb footprint is about 8.25 million.