These are some graphs I created from the data publically available at CiviCRM's Sourceforge. Sourceforge provides limited data only on the release dates of a version (i.e. 3.1.5) and then the number of subsequent downloads to date but we can still interpret some useful conclusions from the data. Dave Greenberg shared one of these graphs at CiviCon. The data is from Mar 1, 2011 so I wanted to publish them before the data became too stale.
The first graphic is the simplest to understand - CiviCRM all versions 2.x compared to all versions 3.x. Note that the timeframe measured in this graph is similar. As such, we can conclude that the number of downloads for 3.x have increased about 27% compared to 2.x.