I saw this discussion go by on the internal PowerShell discussion list and was getting ready to post it but of course Lee Holmes beat me to it.
So, if you are wanting to call PowerShell from cmd.exe (or from the MSBuild <exec> task, which is the same thing), you might want to read his post:
http://www.leeholmes.com/blog/RunningPowerShellScriptsFromCmdexe.aspx
Enjoy!