I'm often find myself reading posts and questions about Data plans, and useage. Well Alan the True blood'n'guts beta testing power user says don't panic.
1. For the last 30 days I purposley went to town downding apps you tube emails BBM etc. And it all went to 364MB
2. When they say a 500MB data plan its an average useage of that, if you go over your allowance once I doubt your carrier would even mention it Vodafone UK don't (they told me). If continually went over you would get a polite email or SMS asking you to refrain or adjust your data allowance. Then if you kept on going over you would be charged.
3. So please don't worry, enjoy your BB. The Data plan is there to ensure a 'fair useage policy' so everyone gets a fair crack of the net.