Sometime You will encounter this problem when trying to Build Android Project on Eclipse.. You will get such output in console:
[2011-05-24 09:25:03 - Android C2DM] Dx 1 error; aborting [2011-05-24 09:25:03 - Android C2DM] Conversion to Dalvik format failed with error 1
You can solve this problem:
Go to Project » Properties » Java Build Path » Libraries and remove all except the "Android X.Y" (in my case Android 2.2). click OK. and then Go to Project » Clean » Clean projects selected below » select your project and click OK.