VMware will run two OS's, but the Guest OS will be reliant on the Host OS. If the Host crashes then it will take down the Guest. If the Host gets infected with a Virus; it can impact the Guest too - since the Guest is reliant on the host.
Why would you want to do this? Firstly, your company could buy you a notebook, any notebook (or you could use your private notebook), and connect to a corporate desktop that is centrally managed. You would no longer have to manage many machines, every time the user starts their machine, they would get a clean desktop image.
Secondly, you may want to run several OS's at the same time, each running on native hardware. You may run Mac multimedia for DTP tools, Linux to securely connect to the internet and Vista for gaming and multimedia. Each OS will now be able to run as if it were running locally with non of the disadvantages of running in a virtual environment, i.e. no multimedia capabilities.
The principle of Virtual desktops is relatively well established and the tools to do this are available - this new technology takes it to a new level....