The "FindRibbons" task could not be loaded from the assembly Microsoft.VisualStudio.Tools.Office.BuildTasks, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a. Confirm that the <UsingTask> declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask
Wow, highlight of my week!
It looks like the issue is related to installing VS2012 side by side with VS2010, it updates the dlls relating to MSBuild, but not the references held in the standard configuration file. The Microsoft forums (via Google) found me my answer in a reply by 'sriram_electric' (thank you!):
In my case I had to go to Program Files (x86) as I'm running a 64bit OS, but other than that, and a quick restart of VS, the problem was resolved.
If this doesn't work for you, there are other suggested solutions given in the thread above.