Yes, it is possible! If your video hardware can run GNOME 3 shell natively, then you should be able to run it in VirtualBox. Your version of VirualBox needs to be 4.0.8 or better. The VirtualBox developers included a patch in v4.08 to make sure it works with then new Gnome 3 shell.
You need to have 3D acceleration enabled and video memory set as high as possible.
You also need to a development kernel (sources and headers) installed on your guest. For Fedora 15, you can install a development kernel as follows:
yum install gcc kernel-devel-$(uname -r)
You also need to install the VirtualBox Guest Additions. The Guest Additions will make your life much easier by providing closer integration between host and guest and improving the interactive performance of guest systems.