For me the "export signed application package" in eclipse always creates an APK but sometimes (right now it feels like more than 50%) when I copy the app on my phone I can't start the app because I immediately get a ClassNotFoundException. I checked it and sometimes classes are missing.
When I then close eclipse and start again and try to export again - it sometimes works.
That's very strange because,
Deleting the bin/dexedLibs Folder did the job for me. Thanks to https://stackoverflow.com/a/16093631/570168