Haha, I also never noticed this until about 3 days ago myself. Actually, it was Ctrl-A (select all) that I first noticed but then saw that ctrl-c (copy) without any text highlighted does the same. I didn't even try ctrl-z until I saw this post.
Now that you bring it up, CTRL+A is a big one that does not work in many places. Try it in a text indicator (won't work). This alone is almost warrants a new Idea.
Undo is still not implemented, but Ctrl-Z no longer writes a "z" (or "Z" for Ctrl-Shift-Z) in VI Properties. Ctrl-X and C with nothing selected also have no effect.