Recently (about a week ago), my favourite terminal shortcut (alt-.) stopped working (it just inserts a literal dot instead of the last argument from the previous command). alt-backspace has also stopped backspacing an entire word at a time as it used to, and now just acts like an unmodified backspace. These keyboard shortcuts still work just fine in guake (a drop-down terminal editor).

I noticed that the mnemonics on the "File | Edit | ..." menus were being underlined when I held down "alt", so I expect gnome / gtk standard menu handling is intercepting the alt key. I disabled "Enable menu access keys" from the gnome-terminal keyboard shortcuts, but that just makes the mnemonics no longer display - it doesn't restore the operation of alt key shortcuts.

It's worth noting that I have a testing version of libgtk-3-0 ( 3.3.5+git20111201.f9c24e8f-0ubuntu1~11.10~ricotz0 ) and other gnome-shell dependencies, installed from:
https://launchpad.net/~ricotz/+archive/testing

(I'd love to remove these to test if it is causing the problem, but I can't figure out how to without apt freaking out)