Take a look at these blog entries:
- http://dennis.dieploegers.de/doku.php/my2cents/handling_keyboard_shortcuts_in_zimbra_zimlets
- http://blog.zimbra.com/blog/archives/2006/09/look-ma-no-mouse-keyboard-navigation-and-shortcuts-in-the-zimbra-collaboration-suite-and-the-kabuki-ajax-toolkit.html
Basically you need to add and register a keymap, than you can define key-handlers.