After over a year on the market, Android Jelly Bean has finally cracked the 40% share of Android. This is really good news for all developers and users. Unfortunately, Ice Cream Sandwich (4.0.x) still has 22.5% share and Gingerbread (2.3.x) still has a 1/3rd of the total Android OS share at 33.1%. Some poor souls (3.9%) are even on versions older than 2.3.
Hopefully before the rumored 5.0 Key Lime Pie in the fall, more people are able to upgrade off Gingerbread and ICS onto Jelly Bean, especially with all the speed improvements 4.3 seems to add with the TRIM technology (especially on the original Nexus 7).
Source: Android Developer Dashboard