How many units of each iOS version are currently in usage?

Marco Arment tells here that his application instapaper had these stats (13 aug 2011)

iOS 4.x: 98.4%

iOS 3.x: 1.6%

(iPad, iPhone and Ipod)

David Lieb (Bump CEO) tells here that the application Bump had these stats for iOS between January 7 and January 10, 2011.

iOS 4.X: 89.73 %

iOS 3.X: 10.25 %

iOS 2.X: 0.02 %

(Note that this includes both iPhones and iPads)

I've found this article on the web : https://blog.jcmultimedia.com.au/2010/08/recent-ios-version-statistics-from-delhi-city-metro

Updated : March 2011

The same company that provided the data for the accepted answer (Chitika, Inc.) has published data for January 2011: 61% of iPads Already Running iOS 4 – January iOS and Android OS Breakdown iOS 3.x for iPhone's share appears to have decreased by 33%!