Woot, I answered my own question. Which is to say, you answered it in your FAQ and I read it:

Need to tag a bunch of tasks at once, or give them the same due date? Multi-edit mode is a special mode that allows you to edit multiple tasks at the same time.

You can toggle the mode on or off by using the shortcut key m.

When multi-edit mode is turned on and multiple tasks are selected, any changes made will be applied to all selected tasks. This makes batch changes quick and easy.

Just in case you or anyone else ends up here referring to this thread, here's the link in the Help page: http://www.rememberthemilk.com/help/answers/tasks/multiedit.rtm

Also, there's a page specifically about tagging multiple tasks: http://www.rememberthemilk.com/help/answers/tags/tagmultiple.rtm

klosterheim, I'm glad you found your answer! :)

I am glad you found the multi-edit. A word of caution... you can really mess things up with multi-edit, so I encourage you to make a real point of using it and, when you are done, turning it off.