First of all, you should have your Android device “rooted” because you need root access to run Ubuntu off your Android.
Second, your Android OS must support loop devices. Most newer Android smartphones/tablets come with this support so we can mount our Ubuntu image as a loop device. If not, you will either have to build your own kernel with support for loop devices or you can try an aftermarket ROM for your phone as most aftermarket ROMs come with support for it.
For example, G2 Phone and Nexus One both come with loop device support so there’s no need for us to compile a new kernel for it but for others, you may have to.
How do you know this? Just try installing Ubuntu and if it doesn’t boot, your Android device needs a kernel with loop device support. Otherwise you are fine.