I'm running Windows 7 64bit as my Host OS and Debian AMD64 as my Guest OS. On my Windows machine a folder called www is mounted on Debian under /home/me/www.
I have no problem installing Plone on Debian (the guest OS) with the unified installer. However, when I try to change the default install path from /home/me/Plone to /home/me/www/plone, the installation always fails because Python fails to install. In the install.log it says 
ln: failed to create hard link 'python' => 'python2.6': Operation not permitted
It looks like it might have something to do with access permissions, but I have tried to run the install script either using sudo or as a normal user, none of it helps. The script installs fine elsewhere, just not in the shared folder in Virtualbox. 
Any suggestions?
More Information: I don't have a root account on Debian (testing, System Python version is 2.7) and always use sudo.
I've done some experimenting with VirtualBox recently. It's great, but I'm pretty sure that the shared folders are going to be limited to what's supported by the host operating system. Windows doesn't have anything like hard or symbolic links.
I suspect that you're trying to do this so that you can edit instance files out of the shared directory with host tools. You might be able to pull this off by installing to non-shared files, then copying the critical parts (like the src directory if you're doing this for development purposes) to a host directory, and then (and only then) establishing that existing host directory as a shared directory.
If you try it, let us know how it works!
Can't Install Plone on VirtualBox Shared Folder because Python Fails to Install