This will not be of much use in calculating the total number of N900s in the wild, but I have made the same app (orrery) available for the Openmoko and Maemo 5 platforms. Two months after the Maemo 5 version was available in Extras it had three times as many downloads as the Openmoko version had had in two years. Of course, only about 20,000 Openmoko phones were sold.