I have searched for this and have found no conclusive answer:

When a calendar invite is received through EAS, typically on a desktop or laptop application of Outlook you can accept/reject/tentative the invite either through the actually email or in the calendar. On the Pre, when you accept of reject the invite it automatically sends an email response to the meeting organizer stating the response (without being able to edit). This doesn't allow the organizer to track your response via the calendar in outlook and also gives the Pre user no way to do that from the native calendar app either.

Is there a workaround for this? A fix coming via an open ticket with Palm? The old calendar on previous Palm models was a godsend and I am frustrated with not being able to do this on the Pre when I don't always have access to a computer on the road.