About every month, Google releases the relative numbers running a version of Android. (See http://developer.android.com/about/dashboards/index.html) Unfortunately they don't provide (a very good) way to see the growth of specific versions. Therefore I'll monitor the page and updates are automatically posted to the table below. This information can be used to decide for what Android version you should develop. Keep in mind this data is global data, and device-usage differs per country. For example, the 2.3 Gingerbread level is much lower in the Netherlands.