I thought I would a get jump on our TRINUG WF project this morning only to find out I could not view any workflows in the designer. I kept getting the error message that a designer for the given file type was not available. After going through the process (several times) of removing the VS .NET Extensions and reinstalling them did I finally come across the answer thanks to Maurice de Beijer's blog post. The installation file for the WF Extensions was the problem. Every time I would start the installation my only option was to do a repair which I thought was odd even after removing it and starting fresh. The solution is to rename the file "Visual Studio 2005 Extensions for Windows Workflow Foundation (EN).exe" to "Visual Studio 2005 Extensions for Windows Workflow Foundation (EN).zip", extract the files to a folder and then running the setup.exe. This will allow you to install the extensions properly and in my case get the workflow designer.
Glad to have this resolved, but I have to say there is simply nothing more annoying than spending a lot of time working just so I can do some work.