Thanks for the response DannySmurf! Unfortunately it doesn't quite help with my situation... I've already seen this solution elsewhere, and it's not quite what I'm looking for.
The solution from dotneverland in particular is risky because it is running from the actual partition rather than a virtual disk. This means if I accidentally boot into Ubuntu, I am hosed... with Ubuntu as my default boot, I would rather not even have the opportunity to accidentally do this.
Also, this requires me to install the newest VirtualBox, but I don't particularly like installing things from outside the regular repositories, so even if running from the actual partition were an option, I would prefer to avoid this solution.
My hope is there is a way to create a .vdi file from the partition, then I can get rolling right away. I'm a bit perplexed why this isn't built into VirtualBox, but maybe they have some good reasons (or they just haven't gotten to it yet).