To ease life of the Android developers who use action bar and tackle the problem of fragmentation between the older devices and the newer devices the following library might be very useful:
github.com/JakeWharton/ActionBarSherlock
The library will automatically use the native ActionBar implementation on Android 4.0 or later. For previous versions which do not include ActionBar, a custom action bar implementation based on the sources of Ice Cream Sandwich will automatically be wrapped around the layout. This allows you to easily develop an application with an action bar for every version of Android from 2.x and up.
P.S. The official Github Android client uses this library.
2015-03-07 UPDATE: Google has released an actionbar component in their compatibility library a while ago. You should be using this now.