Heya! I was away for a few months, I just got into a chat on the official wow forums where a multiboxer claims to have a perfectly working mouse broadcasting tool. (Or hardware means to do so.). He wasn't very specific.

http://forums.wow-europe.com/thread....sid=1&pageNo=3

Quotes from thread:

and to the tauren above who quoted me, its nice to know there are people who actually understand how to box
but you said mouse was poor at best, i disagree, mine works perfectly well. perhaps your rig isnt £3k as i suggested?
and no, i dont have a 3k rig, mine comes to 1800 when new. i found out if you have a harddrive for each wow, it works fine. running off the same HD diff directories are a pain.
Earlier he said that he didn't even use follow in most situations.

Quite frankly, I think he's talking bull, but just out of curiosity, since things could have changed, there still isn't a reliable (ie. PvP situations.) way to broadcast your mouse to multiple screens/wow instances. Right? There wasn't when I left, as far as I know. :P