To be clear, the provocative title to the I/O session is not using the word "hack" in the way it would be used by the mass media with a connotation of doing something you aren't supposed to. Glass is a developer device, and Google wants all sorts of experimentation; so, in this case, the word "hack" is used in the way developers use the term, which is defined perfectly on UrbanDictionary:
The process of installing Ubuntu on Glass took a few steps. First, Glass had to be loaded with Launcher, Settings, and Notepad APKs, then paired to a keyboard and trackpad. After that, it was a matter of jumping into the terminal, unlocking, gaining root access, and following the process you usually would to install Ubuntu. First, load up a Terminal Emulator, and Complete Linux Installer, and you're on your way.
The full session video is below, but the part with loading up Ubuntu can be found at around 5:53:00 or so (we'll link it directly in the source).