VBoxMount is a little tool for providing virtual disk images from VirtualBox as linux block devices. It uses the linux kernel's network block device driver to do it in userspace (otherwise we'd have to bring vbox code into kernelspace which is a no-go).
I have been developing it for Topalis. From the very beginning of the project it was clear that it should become open, and maybe integrated into VirtualBox itself later. It is currently hosted on Launchpad, including all its awesomeness (bazaar branches, releases, bug-tracker).
Grab the first release here. File bugs here. Have fun!
The next item on the ToDo is snapshot support.
P.S.: It is a bit ugly to build right now, mostly because the VirtualBox' SDK doesn't include the necessary headers, nor a package-config file. I will file a "bug" about that at their bug tracker and hope that this will change.
If out-of-tree building doesn't work, you could try the old in-tree branch here. The in-tree version is not as up-to-date as the out-of-tree since I only use the latter one for developing.