[daniel_solano]

Note the: Targeting 4.1 and using the support package gives you full access to Fragments and Loaders while still supporting the older SDK versions.

Sure, if you target 4.1, you can't necessarily use every single new feature in Android on older devices. However, from a design/UI point of view, Android's backports of many new features (and projects like ActionBarSherlock) such as the action bar means you can design for 4.1 and have the code work with many previous versions.

I agree with the grandparent post: design for 4.1 and set 2.2 as a minimum. In cases where you want to use new APIs, you can use conditional classloading techniques.