i was using Eclipse Helios but due to performance issues i changed to Eclipse Galileo and installed the ADT plugging, and added my sdk folder to Elcipse Preferences. Now R.j
same happened to me, corrected checking any errors in res xml files, and changing target 1.5 to 2.2 android (on my case)
this usually happens to me when importing external code or online source code from examples or other developers.