This is adapted from: http://dashes.com/anil/2009/10/how-to-run-windows-7-under-mac-os-x-106-for-free.html
First, install bootcamp and windows as instructed by apple.
Then, open a terminal window and enter:
sudo chmod 777 /dev/disk0s3 sudo VBoxManage internalcommands createrawvmdk -rawdisk /dev/disk0 -filename win7raw.vmdk -partitions 3
Start VirtualBox (download it from http://www.virtualbox.org/wiki/Downloads).
Navigate to your home directory and open win7raw.vmdk.
In your Settings tab in VirtualBox, go to the Storage tab and set the IDE Controller to “ICH6”.
Launch the virtual machine, install the VirtualBox guest additions, and you should be good to go.