Setting Action Bar title and subtitle

There is another option that no one commented, from the Support Library V7 was implemented by Google ActionBarSupport, which is compatible ActionBar from Android 2.2 to the last.
If you change your ActionBar this you can do getSupportActionBar().setTitle("") on all Android versions.

Try this one:

android.support.v7.app.ActionBar ab = getSupportActionBar();
ab.setTitle("This is Title");
ab.setSubtitle("This is Subtitle");

The title for the actionbar could be in the AndroidManifest, here:

<activity 
    . . .
          android:label="string resource" 
    . . .
</activity>

android:label A user-readable label for the activity. The label is displayed on-screen when the activity must be represented to the user. It's often displayed along with the activity icon. If this attribute is not set, the label set for the application as a whole is used instead (see the element's label attribute). The activity's label — whether set here or by the element — is also the default label for all the activity's intent filters (see the element's label attribute). The label should be set as a reference to a string resource, so that it can be localized like other strings in the user interface. However, as a convenience while you're developing the application, it can also be set as a raw string.

You can do something like this to code for both versions:

/**
 * Sets the Action Bar for new Android versions.
 */
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
private void actionBarSetup() {
  if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
    ActionBar ab = getActionBar();
    ab.setTitle("My Title");
    ab.setSubtitle("sub-title"); 
  }
}

Then call actionBarSetup() in onCreate(). The if runs the code only on new Android versions and the @TargetApi allows the code to compile. Therefore it makes it safe for both old and new API versions.

Alternatively, you can also use ActionBarSherlock (see edit) so you can have the ActionBar on all versions. You will have to do some changes such as making your Activities extend SherlockActivity and calling getSupportActionBar(), however, it is a very good global solution.

Edit

Note that when this answer was originally written, ActionBarSherlock, which has since been deprecated, was the go-to compatibility solution.

Nowadays, Google's appcompat-v7 library provides the same functionality but is supported (and actively updated) by Google. Activities wanting to implement an ActionBar must:

• extend AppCompatActivity
• use a Theme.AppCompat derivative

To get an ActionBar instance using this library, the aptly-named getSupportActionBar() method is used.