Just tried to install a soliution, and I received the error:
Install-SPSolution : Microsoft SharePoint is not supported with version 4.0.30319.269 of the Microsoft .Net Runtime.
I just installed the new PowerShell 3.0. To fix, launch a new powershell with -Version 2, and then run your command again:
powershell -version 2
That is what I get for installing the newest and the greatest!