I use a lot of opacity masks these days. It'd smooth out my workflow a lot if there was an entry in the Keyboard Shortcuts that did the same as opening up the Opacity palette far enough to see its thumbnails, and clicking on the opacity mask thumbnail.

Detailed implementation suggestions:

* a menu item in the Transparency palette that either reads "Enter Opacity Mask" or "Exit Opacity Mask" depending on whether or not we're currently editing an opacity mask
* a keyboard shortcuts entry for Other Panels/Toggle Opacity Mask
* NO DEFAULT SHORTCUT, new users would hit it by accident and then be very confused.