Install, Run Ubuntu on Android
Author: Tarandeep Singh on February 12, 2012

Ubuntu installer is the easiest application that helps you install Ubuntu on your Android, without hassles. All you need is a rooted Android having kernel with Loop support. (scroll to the end for list of supported ROMs / devices)

Setup is very simple, the app downloaded from market includes everything you need to install Ubuntu. It endorses a customized Ubuntu 10.10 image optimized for the ARM processor.

The advantage of this Ubuntu installer over other is that it allows you to run a full Ubuntu system within android. i.e. You can still make calls, text, and use android without having to reboot your phone, no dualbooting or multibooting required.

Ubuntu Installers’ Key features:

Run ubuntu within Android.
Access to both Android & Ubuntu at same time.
Highly optimized for ARM devices.
LXDE Desktop (very lightweight)
Ubuntu update manager, Software centre fully operational.
File system access: Ability to access files on your SD card and internal memory from Ubuntu.
Set resolution: Ability to set screen size on boot (you no longer have to download a certain image for different screen sizes!)
Choose Ubuntu Installation type: ‘large‘ and ‘lightweight‘

Large version includes:

Download size: 1.5GB, Installation size: 3.5GB.
UI: Gnome, LXDE
Lots of Apps, idealistic for development: Firefox, Thunderbird, openoffice.org suite, GIMP Image Editor, Emacs Text Editor (geared towards programming), C and C++ build-essential, Java JDK, Python, TeXlive and TeXmaker LaTeX editor, Transmission BitTorrent Client, eVince PDF Viewer, File Manager, Terminal, Image Viewer, Leafpad Text Editor, Synaptic Package Manager and Ubuntu Software Center with all repositories enabled, SSH server, Gnash.

Lightweight image includes:

Download size: 500MB, Installation size: 2.5GB once.
Basic apps required for everything to work.

Source.
Once you have the Terminal App and VNCviewer installed, all you need to do is download one of the Ubuntu images, either the 'large' or 'small' one, and then a small script to mount the image and then start the Ubuntu environment. You can set the desktop size to whatever you want, you don't have to keep it the same size as your phone screen. I VNCed in from my PC to a 1280 by 800 screen, sweet.

Yeah, it's laggy, but hell it's Ubuntu on Android! You can have full-blown Linux apps running, including web browsers.