Canonical, as promised, has made the install images available for Nexus devices. The team has done a commendable job at clearly documenting how to prepare your device for flashing it with Ubuntu and how to go back to Android. You can download images from this page.
Warning: Don't try it on Nexus 4
According to Devices specific issues page, there are some serious issues with Nexus 4. The page states:
In rare circumstances, the Nexus4 may get into a state where it may not boot at all after the battery is drained ( even into recovery ). If this happens, the only way to restore it is to disassemble the back of the phone and unplug/plug the battery connector.
Keep in mind that Nexus 4 has glass back and you can't remove the battery so you many end up with a dead phone. Don't try it on this device unless there are safer images.
It's a preview
Also keep in mind that these are developer previews and not final version so things may not work as they appeared in the Applish video.
it is important to remember that this is in-development software and as such is not finished yet. You are going to find that some features and applications are missing, and you will likely find bugs. We wanted to release the code and images early so that our community can try the software, provide feedback, and be able to join the development effort. With this goal to get the content out early we just want to ensure everyone fully understands that this is not yet a final product. I strongly recommend you only install the code/images on a spare handset/tablet and not your main phone/tablet due to the fact it is in-development code.
Now, go ahead and flash your device.