Small Business Server 2008 / Exchange 2007, fully patched. I swing-migrated from SBS 2003. Replicating the public folder store failed in the migration, so I've had to create some things manually. I haven't messed with any of the system public folders.
Outlook 2003 users can't see free/busy when scheduling a meeting. Outlook 2007 and OWA users can see the info.
I found Problem with Free/Busy aka Availability Service (?) and tried everything in there. There's an EX:/o=<org name>/ou= Exchange Administrative Group (FYBIBOHF23SPDLT) folder and an EX:/o=<org name>/ou=First Administrative Group (FYBIBOHF23SPDLT) folder in the public folder store under System Public Folders\Schedule+ free busy. Autodiscover is working fine. There's no other Exchange servers in the organization (there's a Server 2003 server for running some 32 bit stuff that won't run on SBS 2008). I'm using a GoDaddy cert with no apparent problems.