Is it possible to install Ubuntu on my iPod Touch 2G?
It's possible, but very impractical for several reasons.


Ubuntu is built for Intel x86/x64 officially, and unofficially for the PowerPC (old Macs and the PS3). iPod/iPhone/iPad is built off the ARM architecture, meaning you'd need to do a full rebuild of every single package.
You'd have to exploit the phone to load Ubuntu on to it, and use an exploited bootloader to load anything but a signed iOS.
You'd have to write custom drivers for everything you expected to work, such as the webcam, virtual keyboard, etc.


I'd look into the progress of Android on iPhone. Last I know, Froyo was running alright on an iPhone 3G. Since the full chain of trust has been compromised on all devices except the Verizon iPhone and the iPad 2, it's possible, but the Android port is suffering all the issues I listed above.

Edit: Ubuntu will also expect much more RAM and a much faster processor than the older iDevices have, but the newer devices are more up to par.