SharePoint 2007 workflow activities disabled in Visual Studio 2010 toolbox

Just noticed that when I tried to create a SharePoint 2007 workflow in Visual Studio 2010, the ‘SharePoint Workflow’ toolbox tab was missing. Selecting ‘Show all’ learned me that is was loaded, but they were all disabled.
As it turned out (after comparing the project file with another one where the toolbox was available) I was mssing a ProjectTypeGuid ({F8810EC1-6754-47FC-A15F-DFABD2E3FA90}) in my .csproj file


EDIT: a list of know ProjectTypeGuids can be found here: http://www.mztools.com/articles/2008/MZ2008017.aspx