Dmitry Sotnikov posted a great step-by-step instruction on installing Windows PowerShell on Windows Server 2008 in Server Core mode.
Note that these are in no way official or supported by Microsoft. Microsoft is working on official version of .NET and PowerShell for Server Core installations, so eventually you will be able to get this fully supported. Until then below are the instructions you can use at your own risk, etc., etc.
Source : dmitrysotnikov.wordpress.com,