Like: 
http://connect.microsoft.com/PowerShell/feedback/details/572313/powershell-exe-can-hang-if-stdin-is-redirected
http://stackoverflow.com/questions/4238192/running-powershell-from-msdeploy-runcommand-does-not-exit/4239192#4239192

Another option would be to write the powershell script to a tempfile (securely) and then use `-file <path>`. That might be a good idea anyways, in case the command is long, e.g. from a template, and exceeds the maximum CreateProcess length. Thoughts?