Adding events to calendar - default calendar for entry

First off - we have several "calendars" of events (1 for each family member) and we have them all displayed (checked off) on iCal and iPhone. Sync's all great under mobileme etc...

Now when my wife enters an event into her iphone it defaults to my son's calendar and she has to manually change the color (calendar) to hers. Is there a way to make her calendar the default calendar for her iPhone? minor inconvenience if you ask me, but I can see the point if there is a default calendar somehow why not make it hers since that's where she inputs most if not all of her calendar appts.

Thank you all in advance!