Turns out it's not debootstrap. It's the subsequent apt-get dist-upgrade with oneiric-updates and oneiric-security added to /etc/apt/sources.list that's leaving the mount behind.
Turns out it's not debootstrap. It's the subsequent apt-get dist-upgrade with oneiric-updates and oneiric-security added to /etc/apt/ sources. list that's leaving the mount behind.