I just completed a migration from Exchange 2003 to Exchange 2010 with 95 users now all on the 2010 environment. I have a mix of users on Outlook 2003 and Outlook 2007. Of those 95 users, I have 2 where free/busy time shows as unavailable to users of Outlook 2007 or OWA, but Outlook 2003 users can see their free/busy just fine. Of the two, one uses Outlook 2003 and one uses Outlook 2007.
Aside from those 2 for whom free/busy is only viewable in outlook 2003, all other users' availability shows up fine regardless of whether Outlook 2003 or 2007 is used.
I did have some pains migrating our public folders to 2010, and I have to sort out a Sync Issue that's likely related to our Organizational forms. But OAB and Free/Busy for other Outlook 2003 users works fine, and one of the users is on Outlook 2007, so I'm not convinced that it's entirely a server issue. Any suggestions?