How large is the QGIS community? I'm interested in understanding the shift from traditional proprietary software to open sourced solutions.
Download stats are one way to look at it: http://linfiniti.com/2011/10/qgis-1-7-0-download-stats/
There is also a user map if you are interested in geographic distribution of users http://plugins.qgis.org/community-map/
But all these can only capture a small part of the whole community.