For legal reasons,PowerShell RTM on Vista RTM is not yet available. However, for an enterprising techie, hand installing is not a big deal as this blog post shows. Basically, you copy over code from and XP system and do some registry hacking.
Naturally this is NOT supported, nor is it particularly recommended even if it does work!
And if you are a PowerShell Fan in the UK, pencil in the evening of January 24th. More details to follow.