In Outlook, have you tried to select All Public folders, then Tools, Recover Deleted Items? By default Deleted Item Retention is set to 14 days in Exchange 2007 (Public Folder Database Properties, Limits, Keep deleted items for days).
You could also try with the PFDAVAdmin tool (the steps here do also apply to Exchange 2007) How to recover deleted public folders or items that are deleted from public folders by using the PFDAVAdmin tool in Exchange 2000 Server or in Exchange Server 2003 http://technet.microsoft.com/en-us/kb/kb00924044.aspx
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