We have an Exchange 2010 native deployment, which was upgraded from Exchange 2003.
The problem is that Outlook 2003 users cannot see the resource (room mailbox) free/busy. (No free/busy information could be retrieved)
Outlook 2003 can see other users F/B.
Outlook 2007 / 2010 / OWA can see the resource F/B.
I have had a look around and can only think that the MS exchange system attendant is not pushing calendar updates into the F/B public folder just for the resource mailboxes.