I have been having problems with version 2012.2.608 of JustMock. TFs build definitions which make use of the JustMock workflow activities fail instantly. Installing the previous version of JM fixes this issue.
What I think is happening is that the Telerik.JustMock.Build.Workflow.dll Assembly now has a dependency on Microsoft.TeamFoundation.Build.Client Version 126.96.36.199 which is part of TFS 11 which I think still is in beta. We don’t have this version of the assembly so the build server fails instantly. Using Reflector I can see that the previous version of JM depended on Microsoft.TeamFoundation.Build.Client Version 10.0.0.0
Please look into this.