How to install Ubuntu from OS X? I'm looking for a solution that would enable me to install Ubuntu and run it as a virtual machine under OS X but still be able to boot directly Ubuntu.

If possible I would like to test both scenarios: Ubuntu guest on OS X host and Ubuntu host and OS X guest.
If I remember correctly, you should be able to install Ubuntu into Boot Camp, and then virtualize it using Parallels Desktop. Works great with Windows; most likely with Ubuntu as well.