Hello Mark McPherson,
First of all are you sure that you have created SL4 project and not SL3 as usually you are getting the exact same error in this case. Also do you have our SL3 controls installed on the machine? There is some conflict when you have our SL3 controls installed and trying to use our SL4 assemblies and the error was similar.
Hope this helps. If the problem is still there please let me know and some info about VS 2010 version would be of help, is it RC or Beta2 etc.
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