This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
It is a convention - not a law. NO ONE can quarantee that others won't use it. However, since the industry standard is to use package names with an inverted domain address that you have registered, no one else would be able to register the SAME domain name. Therefore if everyone follows this standard there will be no clashes. So all packages belonging to GM would be in packages named com.gm.whatever etc.
"JavaRanch, where the deer and the Certified play" - David O'Meara