The increase is the result of new handsets such as the HTC One X and One S smartphones, which ship with the OS, arriving on the market, as well as updates being released for existing devices like the HTC Sensation, Samsung Galaxy S II and the Nexus S 4G on US CDMA carrier Sprint. The most popular version is still Android 2.3.x, known as "Gingerbread", which increased from 63.7% of all devices to 64.4% – new phones, such as the Sony Xperia S, are still being released with Android 2.3.x six months after version 4.0 was announced, but these are expected to be upgraded in the next quarter.
Source: developer.android.com
Usage of older versions of Android continues to decline: Android 2.2 "Froyo" now accounts for 20.9% of all devices, down from 23.1% in April and 27.8% in February, and version 2.1, known as "Eclair", is down to 5.5% from 6.0% a month ago. However, usage of Android 3.x "Honeycomb" tablets, and devices running 1.5 (Cupcake) and 1.6 (Donut) has remained the same as last month at 3.3%, 0.3% and 0.7% respectively.