Just had a learning experience.

Virtualbox-ose (4.04) on Ubuntu 11.04 will not allow the Fedora 15 Guest Additions to Install successfully.

I had to download Virtualbox-4.0 (4.08) from from the http://www.virtualbox.org site and the guest additions installed and worked fine.

Now I have a Fedora 15 Virtualbox guest running Gnome3 successfully.