Update 9/19/13: A side-by-side coexistence of PowerShell 3.0 and PowerShell 2.0 is supported starting with Exchange 2010 SP3. Note that Exchange 2010 will only use PowerShell 2.0 so PowerShell 2.0 is still a required component.
Recently, Windows Update began offering the Windows Management Framework 3.0 as an Optional update. This includes all forms of update distribution, such as Microsoft Update, WSUS, System Center Configuration Manager and other mechanisms. The key bit here is that the Windows Management Framework 3.0 includes PowerShell 3.0.
Windows Management Framework 3.0 is being distributed as KB2506146 and KB2506143 (which one is offered depends on which server version you are running - Windows Server 2008 SP2 or Windows Server 2008 R2 SP1).
Windows Management Framework 3.0 (specifically PowerShell 3.0) is not yet supported on any version of Exchange except Exchange Server 2013, which requires it. If you install Windows Management Framework 3.0 on a server running Exchange 2007 or Exchange 2010, you will encounter problems, such as Exchange update rollups that will not install, or the Exchange Management Shell may not run properly.
We have seen Exchange update rollups not installing with the following symptoms:
Product: Microsoft Exchange Server - Update 'Update Rollup 5-v2 for Exchange Server 2010 Service Pack 2 (KB2785908) 14.2.328.10' could not be installed. Error code 1603. Windows Installer can create logs to help troubleshoot issues with installing software packages. Use the following link for instructions on turning on logging support: http://go.microsoft.com/fwlink/?LinkId=23127
Our guidance at this time is that Windows Management Framework 3.0 should not be deployed on servers running Exchange 2007 or Exchange 2010, or on workstations with the Exchange Management Tools for either version installed. If you have already deployed this update, it should be removed. Once you remove the update, functionality should be restored.