I got the error Server Error in '/' Application. ________________________________________ Could not load file or assembly 'ESRI.Solutions.WMServer.Handlers' or one of its dependencies. The system cannot find the file specified.
After applying the changes in here and here, and changing the Classic .NET AppPool Managed Pipeline Mode to Integrated, I still needed one extra change.
My wmrest folder needed to be converted to an application. Apparently the install should have done this; it didn't (no idea why not, I was admin, only other software on the box was ArcGIS).
We found this issue with our 10 final and 10 SP1 versions and made a series of enhancement to Workflow Server at SP2 for ArcGIS Server .NET users. Please install the service pack at your earliest convenience.