I've recently been asked whether Windows SharePoint Services 2.0 can be upgraded to Microsoft Office SharePoint Server 2007; while this is possible, there are several important constraints that should be taken into consideration.  Foremost, you should not upgrade Windows SharePoint Services 2.0 to Microsoft Office SharePoint Server 2007 bypassing the Windows SharePoint Services 3.0 upgrade - by attempting to upgrading Windows SharePoint Services 2.0 to Microsoft Office SharePoint Server 2007 you are combining two upgrade methodologies; version to version (WSS 2.0 -> O12) and SKU to SKU (WSS 3.0 -> O12).  While possible in practice to achieve success by implementing this upgrade framework, it is both untested and unsupported, which should be taken into consideration when applying future QFE's and service packs and what impact will result as a result of the state of your environment.  An example of this statement is that setup sets a Registry key indicating sku2sku upgrade, PSConfig recognizes the key and subsequently runs all of the Microsoft Office SharePoint Server 2007 upgraders on the farm which provides a true SKU to SKU upgrade.  In a Windows SharePoint Services 2.0 to Microsoft Office SharePoint Server 2007 upgrade, it is untested in whether or not those upgraders are run.  In the event the upgraders fail to run, Microsoft Office SharePoint Server 2007 version information will be missing on the Site Collections in their associated Content Databases impacting future upgrades and patching.  With that information in mind, the supported and tested upgrade path is Windows SharePoint Services 2.0 to Windows SharePoint Services 3.0 and then Microsoft Office SharePoint Server 2007.  This upgrade method will ensure the required upgraders are run as intended.  One of the most common applications of this upgrade method is an in-place upgrade of Windows SharePoint Services 2.0 to Windows SharePoint Services 3.0 followed by the installation of Microsoft Office SharePoint Server 2007 over the Windows SharePoint Services 3.0 installation - at this point the databases can be upgraded using the supported and tested SKU - SKU method.