Hi Matt,
the popup message must be "<resource name> is already booked for the specified time. You must use another time or find another resource.".
After few research i found this post: http://social.technet.microsoft.com/Forums/en-US/exchangesvradmin/thread/2fc784c8-d2ef-4438-96e1-08918f5c0e12/ . And I've test the following configuration: "connect to Exchange as the resource mailbox, go to Tools, Options, Calendar Options, Resource Scheduling and check the first two options ("Automatically accept meeting requests and process cancellations" and "Automatically decline conflicting meeting requests")." and yes I've the popup.
With this configuration the organizer will receive immediate popup response instead of a message.
Murat