Your best bet is going to require a significant amount of manual work. Technically, you can't Boot Camp Ubuntu as it is a specialized way to streamline Windows dual boot installs on Macs.
However, you can use it to create an initial partition, which you delete and format for Ubuntu's use. Alternatively, you can use Disk Utility to decrease the size of your OS X partition. Then use Ubuntu's installer to modify that empty space into usable partitions for Ubuntu. From there, install Ubuntu as usual. (Be sure to hold down C while booting to ensure you boot from the disc.)
Finally, you would have to choose your virtualization platform:
- VirtualBox supports raw disc access but has very large warnings against its usage due to data corruption concerns, etc. Chapter 9 of their manual covers raw disc access.
- VMware Workstation supports raw disk access, but unfortunately doesn't work on OS X. Even worse, VMware Fusion doesn't appear to support setting up raw disk access from the GUI.
The workaround, should you choose the VMware approach, is to use the command line tools and manually editing config files to give you access to raw disk access. This forum thread has useful discussion on this process, as well as other concerns.