Make sure and upgrade to B2TR for production WSS V3
Joel points out that you will HAVE to upgrade to Beta2 Technical Refresh (B2TR) if you want to upgrade to the RTM version of SharePoint V3. Make sure you plan and stage your production testing environments and as always back everything up first.
Full link: http://blogs.msdn.com/sharepoint/archive/2006/07/14/665864.aspx