The folks at Google keep track of the version of Android that's running on every device that accesses the Android Market. Every 14 days, they calculate a breakdown of the users by percentage of the devices running each different version of the Android API levels which we mostly think of as Android versions. See the table below for specifics.
As of Monday, 2/20/2012, nearly two-thirds (58.6%) the active Android devices are running Gingerbread, and almost all of those Gingerbread phones (58.1% of all Android phones) are running API Level 10, released with version 2.3.3 in February of last year.
|Android 2.3Android 2.3.2||Gingerbread||9||0.5%|
|Android 2.3.3Android 2.3.7||10||58.1%|
|Android 4.0Android 4.0.2||Ice Cream Sandwich||14||0.3%|
There are seven different platform names, from Cupcake to Ice Cream Sandwich, and there are 11 different API levels over those platforms. Earlier versions of Gingerbread (prior to the 2.3.3 release in February of 2011) had a different API Level than all subsequent Gingerbread releases. Honeycomb's actually had three separate platform releases with an update to the API Level on each one.
The second largest share is the 27.8% of devices running Froyo, first introduced in May of 2010. Over 86% of devices are running either Froyo or Gingerbread. According to a graph of historical distribution of versions available with the Android report, there was a fairly rapid switchover from Froyo to Gingerbread. Last August, only about 25% of devices were running Gingerbread, and more than 50% were running Froyo. By about the middle of November, Gingerbread crossed the 50% mark.
It's almost never users who decide which version to run, but the carrier from whom they buy their phone. There are several likely explanations for the time lag between the release of a newer version of the platform and its broad appearance in the field. It may take the network providers some time between receiving a release of Android and the time to integrate the provider's code into a releasable product (Motorola's claimed that reasoning for the delay in releasing Ice Cream Sandwich upgrades for new Droid4). Ice Cream Sandwich is better-suited than earlier versions to both phones and tablets, but is too new to show any significant market share.
Because the raw data are not made available, it's difficult to tell how much of the change comes from the release of new phones running Gingerbread, and which portion came from Froyo-to-Gingerbread upgrades. I suspect that the raw data that we would need to say for certain are proprietary to carriers.
It's also interesting to note that only about 3.4% of all Android devices are running Honeycomb, and that that's been the percentage since about mid-November. Ice Cream Sandwich appears on only 1% of devices.
Of course, the information only tracks users of Android Marketplace, however it's hard to imagine that someone would go to the trouble and expense of getting an Android smartphone and never use Marketplace. In fact, given automatic checks to make sure that various apps are up to date, it may be impossible! The percentages are probably an accurate look at active devices on Marketplace. However, it would be interesting to know if there's any way for the data to include counts of devices like the Amazon Kindle Fire , which uses a custom version of Android 2.3, or of Android devices sold in China which aren't allowed access to Marketplace at all.
Based on the data that I can see from market.android.com when I look at my account, Google knows all of the devices that I have ever used Marketplace with, and knows when they were each last active. There are probably interesting reports on that sort of data that might let you know how many older phones were activated but went dormant for some reason or another... for Android's sake, one hopes for an upgrade!