So, what did I do with it? I could have made a gazillion interesting and useful programs, but I decided that for a proof-of-concept, something more whimsical would do just fine:
If you have a Zx3 (ith firmware version 1.12), you can try playing Tetris yourself. Just ungzip/untar this file, copy autoexec.ash and hackapp.bin to the root of an SD-card, insert card into Zx3 and do what I did in the video. The hack won't damage your camera in any way: all that's modified is the code running in RAM and when you remove both files from the SD-card and reboot, the hack will be completely gone.
As usual: the source code for the hack is downloadable here and licensed under the GPLv3. Do with it what you want: stuff like a full-on CHDK-suite for Ambarella-based devices is in theory possible. I'm probably not gonna start that, though: I have my DSLR for pictures and N900 for image hacking, so I'm happy with only recording HD-movies with my Zx3... and playing the occasional game of Tetris ofcourse.