Note that this question is mostly so others can find this information.
I ran into a problem recently with my Fedora-14 virtual machine running on a Windows-7 host with VirtualBox.
VirtualBox could not start the Fedora virtual machine, but it could start a Ubuntu virtual server machine that was configured for 512 MB of RAM. The Fedora-14 machine was configured for 2 GB of RAM.
The Fedora-14 machine had been working a week ago. What had I recently changed...?
Answer to follow.