As an early Christmas present, Canonical have unwrapped the greatest feature yet with their mobile OS. Ubuntu on mobile devices now support dual booting with Android. They are not recommending it for all users, rather just the developers or advanced users that are comfortable with flashing their devices.
In order to get dual boot working, the Android recovery partition will need to be rewritten so definitely not for the novice. AOSP/stock Android or CyanogenMod are confirmed as being supported.
Instructions on installing dual boot can be found on their wiki page.