Many weblog systems offer this for their entries, under the same principle: the blogging interface is designed to usually deal with entries on a singular basis, but recognizes that occasionally "mass changes" may want to be made, and offers an interface for same.

In my case, for example, I am thinking about redoing the tagging system I've adopted. However, in order to do that, I'd have to be repeating a HECK of a lot of keystrokes to get to the point where I can manually edit each task's tags.