Summary

Windows Workflow will produce a radical change in the way that applications are constructed. You can now surface complex parts of an application as activities, and permit users to alter the processing of the system simply by dragging and dropping activities into a workflow.

There is almost no application that you could not apply workflow to - from the simplest command-line tool to the most complex system containing many hundreds of modules. While the new communication capabilities of WCF and the new UI capabilities of WPF are a great step forward for applications in general, the addition of Windows Workflow will produce a seismic change in the way that applications are developed and configured.

If you only have time to invest in one of the new facilities available with the .NET Framework 3.0, I would suggest concentrating on Windows Workflow. I would expect skills in workflow to be very highly sought after for years to come.