If you are having problems with free/busy information in Outlook, it is most likely due to misconfiguration of the Exchange 2007 Autodiscover service.
The Autodiscover service provides info to the Availability service, such as the addresses (internal and external) that Outlook 2007 clients should use to connect to Exchange. More info here.
I would recommend that you read this post at the Microsoft Exchange Team blog and the referenced whitepaper at the top.
Most likely, you do not have the internal/external addresses configured correctly in Exchange. Double-check these. In addition, with Outlook open, you can hold the ctrl key and right-click on the Outlook icon in your system tray to get the “Test Email Autoconfiguration” option. Run this to see how Outlook is trying to connect to your Exchange server. You may notice that Outlook first tries to connect to “domainname.com”, then to “autodiscover.domainname.com”. These are the default addresses that Outlook tries. You may need to create a CNAME for “autodiscover.domain.com” which points to your Exchange proxy address. That is the address that you have in the Exchange proxy connection settings in Outlook, and is likely the same as your OWA address.
Once you have the Autodiscover/Availability services configured correctly, you will likely find that your free/busy info problems have been resolved.