Rather than hearing directly from Microsoft, I indirectly found this supportability through several SharePoint workflow project templates in Visual Studio 2008. Visual Studio 2008 comes with several project templates for creating workflows, and these workflows are based on .Net 3.5.

There are many benefits of using .Net 3.5 for WSS 3.0 and MOSS 2007 (or SharePoint) esp. for WCF and workflow. WCF and workflow in .Net 3.5 come with many new features and they are tightly integrated with each other. For example, it's now possible to call WCF services inside a custom SharePoint workflow using ReceiveActivity and SendActivity as well as inside a custom workflow activity. Instead of replying on traditional web service for communication with external systems, SharePoint can now leverage WCF even inside custom workflows for more reliable messaging.