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.
Now, as a boon to multiboxers everywhere, former Wowhead CEO Tim Sullivan has released MouseCloner. To quote his site, "Mousecloner is an application that enables you to 'replicate' mouse commands on your desktop with the simple pressing of a single key. The pressing of 1 keystroke can activate single mouse movements and clicks on seperate instances of World of Warcraft." It follows in the spirit of the "one action per keypress per account" policy that Blizzard has publicly expressed many times.
What's refreshing about this as compared to a lot of other multiboxing aids is that Mr. Sullivan has made a great effort to make sure that every function of the program is in accordance with Blizzard policies. He's even compiled a list of Blizzard responses on the website for those who're concerned about its validity, and states that if a feature of the program comes under fire by Blizzard at any time, they'll immediately remove it.
I'm not a multiboxer myself, surprisingly -- I'm pretty universally terrible at it -- and I haven't tried MouseCloner, but having a developer focused both on obeying Blizzard policy and helping players is a nice change and I hope he gets a lot of support for it.