Not possible for Parallels, VMWare or anyone else who is a company because Apple expressly disallows it. VMWare was asked if this would be a possibility for their upcoming product and they indicated they'd love to do it, and that it would be easy, but unless Apple changed their policy it would not be happening. I suspect the same is true for Parallels. People need to beat on Apple if they want this.
I'm sure there will be unofficial hacks based on the ability to install OSX on arbitrary Intel boxes at some point, but that won't be a official/legal solution.
