New cabrio 4/19/2008 - love it. Wife went out to drive to work this AM and it won't start.

Everything electric seems okay, but engine doesn't turn over. Service Mgr says jump it or tow it in. I put on battery charger for 3 hours, now everything works fine.
Problem is ... what happened? will it happen again? what can I do to insure that it doesn't happen again?
Possibility ... we're in Florida, it's hot, AC all the time, radio loud, fan blowing, lights on, etc. Is that the problem? What to do?
