How to Switch to Ubuntu
Ubuntu.com has a few good articles on how to make the switch to Ubuntu Linux.
How to Switch from Windows to Ubuntu Linux
The guide for Windows users explains some of the differences between Windows and Ubuntu Linux.
It recommends running an Ubuntu live CD first to make sure that Ubuntu works with your hardware.
If you are going to switch from Windows to Ubuntu, I recommend trying a dual boot system first. A dual boot system allows you to run Windows and Linux next to each other on the same computer. Ubuntu makes installing a dual boot system easy.
How to Switch from Mac to Ubuntu Linux
If you are switching from Mac to Ubuntu it is less of a leap than switching from Windows to Ubuntu. Mac OS X is also a Unix-based operating system and it is similar to Linux on the inside.
Mark Pilgrim lists some of his tips for switching from Mac to Ubuntu on his blog. It's an interesting discussion for anyone who is making the switch to Ubuntu.
How to Switch From Another Linux Distro to Ubuntu Linux
This is the easiest switch. Ubuntu is one of the easiest to use Linux distros and you shouldn't have any problems switching to Ubuntu.
There is an interesting new blog called UbuntuSwitch.com.
The Ubuntu Forums are a good resource of information about Ubuntu.
The Official Ubuntu documentation is a free 80-page ebook that can help you get started with Ubuntu.