Serge,
I think it would be even better if one didn't have to use a mouse and could add keywords with keyboard shortcuts.
One approach would be something like "Ctrl+K L" where Ctrl+K would signify a keyword shortcut and the next letter -- in this case "L" -- would signify a user defined keyword (perhaps Landscape).
These keyword shortcuts should be accessible in the contact sheet, preview window, and IPTC info panel. This would allow you to add keywords in any mode using a consistent interface.
Deven