I use the following script to boot my Windows 7 bootcamp partition under OSX with VirtualBox. Replace the name of your Bootcamp partition and the name of your VirtualBox image accordingly. The script was written after following this description: http://www.kevinrockwood.info/2010/04/windows7-in-osx-with-bootcamp-and-virtualbox/

vb_bootcamp.sh

#!/bin/sh

# find and unmount volume (change 'BOOTCAMP' name accordingly)
amount=`diskutil list | grep BOOTCAMP | wc -l`
if [ $amount = 1 ]
then
  path=`diskutil list | grep BOOTCAMP | sed -e 's/^\(.*\)\(disk0s.\)$/\/dev\/\2/g'`
  sudo chmod 777 $path
  diskutil unmount $path
fi

# start virtual machine (replace 'WindowsBootcamp' with your VirtualBox image name!)
vboxmanage startvm WindowsBootcamp