The latest VirtualBox 4.0.8 finally comes with GNOME Shell support. I've tested it in Ubuntu 11.04, using a Fedora 15 virtual machine and Gnome Shell worked great. According to the changelog, it should work with both Ubuntu 11.04 and Fedora 15:
3D support: fixed GNOME 3 rendering under Ubuntu 11.04 and Fedora 15
You can find the complete VirtualBox 4.0.8 changelog here.
How to get GNOME Shell (in Ubuntu 11.04 or Fedora 15) to work in VirtualBox
To get GNOME Shell to work in VirtualBox 4.0.8, install VirtualBox (Ubuntu installation instructions), then select the virtual machine and in the settings check the "Enable 3D acceleration" box (under Display, on the Video tab).
Then start Ubuntu 11.04 (make sure you add the Gnome 3 PPA and run an upgrade to get the latest packages if you want to run Gnome Shell VBox) or Fedora 15 beta and on the machine menubar select Devices > Install Guest Additions.
Note: for me, the AppMenu (Global menu) doesn't work for VirtualBox 4.0.8 in Ubuntu 11.04 (host). To get a working menu, launch VirtualBox like this:
Now you must install the Guest Additions - you should be prompted to install once you boot either Ubuntu or Fedora if you've followed the steps above (if not, open Nautilus and you should see VBOXADDITIONS in the sidebar - clicking it, you should see an "Open Autorun Prompt" button). For Fedora 15 you must install some additional packages - see this page.