I'm in the middle of an Exchange 2007 migration and the Outlook 2007 clients with Exchange 2003 mailboxes cannot retreive free/busy data for some of the Exchange 2007 users.
Two Exchange 2007 mailbox servers. One 07 server has a PF database and the other 07 mailbox server does not.
The Exchange 2003 users can see free/busy data for the 07 users on the server that has a PF DB but they cannot see free/busy data for users on the second 07 mbox server.
The Exchange 2007 users on both mbox servers can see everyone's free/busy data without issue.
I set the mailbox database on the second 07 server to use the PF database on the first 07 server.
Seems like the free/busy data from the second 07 server is just not getting to the free/busy folders so that Exchange 2003 clients can see it.