actionbarsherlock.com/faq.html
>What API level should I compile with if I want to use the library?
Use of this library requires that both it and your project are compiled with Android 4.0 (API 14) or newer.
>What API level should I target in my manifest when using the library?
Targetting API level 11 or newer is required as it will cause Android to automatically add the native action bar when run on newer devices. Since you will be compiling against new APIs but your app will likely be run on devices with older versions of Android extra care must be taken to either avoid using or properly check and call any methods that were introduced after your minimum SDK version.