Update: Despite evidence that Blizzard was okay with this app, on the Customer Service forums, Malkorix tells users he "strongly advises shying away from this program." Use it at your own risk, folks.
Update II: The folks behind MouseCloner have informed us that while the app did originally use "prerecorded x/y coordinates" (which is what Malkorix found a problem with), they have specifically recoded the app to use only mouse position, in order to stay away from questions about scripting. To our (and their) best knowledge, the app is safe to use, but as always, you use third-party programs with your client at your own risk. You can find more information and ask more questions on the Mousecloner forums.
Original Post: Now here's a subject with my name written all over it. Those of you who followed my exploits over on the official forums will likely remember my responses to multiboxing questions with a nice, clear stance on what Blizzard does and doesn't support with regard to multiboxing tools. It was good for the multiboxing community -- and bad for anti-multiboxing trolls -- to have readily-available blue responses to those kinds of important questions. Even Tom Chilton (a.k.a Kalgan) has come out in support of the practice.
For those of you unfamiliar with it, multiboxing is the act of playing more than one WoW account/client at the same time. There have historically been a number of programs used by multiboxers that help with client-switching or keybinding to make controlling two or more accounts easy.