Scott Adams wants one for his smart phone:
The biggest problem with a list, especially once it gets to a dozen items or more, is that a list is one-dimensional. Ideally, I want my list sometimes organized by priority, but other times by location. For example, my to-do list app should sense my speed and motion and sort to the top of the list any tasks that involve phone calls, under the theory that I'm probably driving my car and I can make some calls on the way.
Other times I want my to-do list sorted by location. If I'm driving past the store, the items I need from the store should sort to the top of the list automatically. That function already exists in at least one "notes" app I've seen. At other times I want my list to have the simplest and quickest items on top because I might have a spare five minutes and want to knock off a few items.