Like already said, dpkg has locked the package database. Usually this means there's already another dpkg process running, background or foreground. If you don't have Application Manager running and yet you still always get this message, there's something wrong. Killing dpkg-processes or removing (a stale) lock file might help for time being but it's hardly the solution. apt-worker should check for updates only so often -- it shouldn't be running all the time.

What does "ps | grep apt" return?