OsSwitching
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Found Footage: OS switching on a MacBook
This video is a little longer than it needs to be to make its point, but it is a good illustration of how the MacBook can handle emulating 2 OSes whilst running OS X (thanks to Parallels).
Found Footage: Fast OS switching on an Intel Mac
This video has been making the rounds, and since it is so cool I thought I would bring it to your attention. Using a combination of Parallels and Virtue Desktops this person as able to setup their Mac mini to 'switch' OSes from OS X to Windows XP to RedHat.