After finally getting my machine setup back in order after and during the Agile 2006 conference, I realized that I hadn't updated my Monad/MSH syntax file to support PowerShell (ps1).
The updates are done and I have uploaded the file to vim.org. FWIW, the changes are almost none... just a replace of "msh" with "ps1" and all is well. After I use it some more, I will update it as needed.
I will post some LUA-friendly installation instructions shortly, but for now you can use my old instructions (not-LUA friendly).
Enjoy!