15 Minutes seems to be the magic number for a meeting reminder default when you put Outlook 2003 in an Exchange 2007 environment.
Expect that under a variety of conditions of meetings being proposed to and from OL2003 that the default will be set to 15 minutes on the ATTENDEE's calendar, regardless of where it was set in the meeting ORGANIZER's, and regardless of the ATTENDEE's Default reminder setting.
The situation arises with default settings and reminders if Outlook 2003 is the origination point. We haven't (yet) seen problems with OWA or Outlook 2007.There are in the support database a variety of problems listed with meeting reminders in Outlook (appointments seem to be working OK, but maybe that's just our eyes glazing over).
Among the classics:
The reminder for the meeting is removed when you accept a meeting request in Outlook 2003 which is based on Rules removing reminders
A reminder for an all day event meeting request is reset to 18 hours when you send the meeting request in Outlook 2003 which is self-explanatory.
Try just hitting the support.microsoft.com and searching for "Outlook reminders".