While the slow adoption of Android 4.0 Ice Cream Sandwich continues, Jelly Bean is also picking up speed. The latest numbers directly from Google show that 29.1% of devices that recently accessed Google Play are running Ice Cream Sandwich. 4.1 and 4.2 Jelly Bean combined are now on 10.2% of devices. Jelly Bean has passed the 10% milestone a few months earlier than Ice Cream Sandwich did, which was in July when the Nexus 7 was released.
The rate of adoption is so fast probably because Samsung has been pushing Jelly Bean to its flagship phones.
(Source: Android Developers)