Under Preferences, choose Java –> Editor –> Save Actions. Check the Perform the selected actions on save, and check the Format source code box.
This may or may not be available in previous versions of Eclipse. I know it works in:
Version: 3.3.3.r33x_r20080129-_19UEl7Ezk_gXF1kouft<br>
Build id: M20080221-1800