I've only seen a couple of reports of such errors. At first I thought you were trying to build the solution (never could get that to work) instead of just the WPF project maybe, but the errors are quite different when I try to build the whole solution.
A couple of things to try:
Clean the solution, then build the WPF (or SL) project. I think you need the Blend SDK (and I think it's a different one for WPF and for SL) installed by the way. Most likely won't do anything, but it takes a minute to try it.
If that doesn't work, delete the entire directory structure where you unzipped Cinch, then unzip it again, then load the solution and try to build the WPF (or SL) project - just make sure you have all the pre-requisites.
The only other solution I saw involved having to edit the IL to change the namespace. That sounds "way out there."