To configure a resource mailbox to automatically accept an appointment in Exchange 2007, you must use the Exchange management shell.
If there is already an appointment that overlaps at all with the appointment you are trying to schedule, you will get back a ‘Declined’ receipt from the resource mailbox.
Assuming you already have the mailbox created, and it is called “ResourceMailboxName”, here is the command you should run:
Set-MailboxCalendarSettings ResourceMailboxName -AutomateProcessing:AutoAccept
See these pages for more info on resource mailboxes: