Resource mailboxes do not autoaccept requests from users in a subscribing forest. Calendar requests are added to the Resource Mailbox Calendar but are left with a status of "Pending"
By Default Exchange Resource Mailboxes will not autoaccept meeting requests from users outside the exchange organization. Users in a subscribing forest would fall into this category.
For resource mailboxes in an Exchange 2010 or 2007 Organiation this functionality can be enabled on a per mailbox basis using the following cmdlets:
Exchange 2007 SP1 and higher:
Set-MailboxCalendarSettings -Identity "Resource Room Name" -AutomateProcessing AutoAccept -ProcessExternalMeetingMessages $true
Exchange 2010:
Set-CalendarProcessing -Identity "Resource Room Name" -AutomateProcessing AutoAccept -ProcessExternalMeetingMessages $True
