I am running exchange std server 2003 SP2 with outlook 2002 SP3 clients. The clients are receiving their calendar reminders late. For example, the time of the meeting passes and then the 15 minute reminder might display hours or a day later. Any ideas?

Have you tried restarting the Exchange services? Almost sounds like your System Attendant is not working correctly. Did you recently delete the mail store that contains the System Attendant?