I can see two schools of thought here in how they relate to multiboxing and how you would broadcast mice.

The first school:
I press a hotkey and the mouse click replicates to one/some/all clients, where my mouse currently is on the window I am controlling.

Mechanics Involved: To accomplish this with hardware you quickly move a specific mouse to a corner then move it back to where you want to aim. With practice this movement becomes second nature and a click is executed at the same location on all PC's. To accomplish this with software the software moves the mouse on the clones to the current mouse location on the main, and broadcasts the click. The alternative is that the software ALWAYS updates the clients mouse pointer position but only broadcasts clicks on the hotkeys. (If you turn on the virtual mouse cursor in IS you will see this behavior demonstrated.)

My thoughts? Hardware is the purest form of broadcasting mice, it does have limitations as you have to zero the mouse and you (practically speaking) need to have identical resolutions on all machines. Additionally you have a chance of it coming unsynced. Software this is as close to hardware as you can get and avoids anything that may be seen as a form of automation.

The second school:
I press a hotkey and the mouse click sends to some/one/all clients at a pre-determined position which I cannot easily change on the fly.

Mechanics Involved: This is the equivilant of having individual mice setup across machines and having them pre-positioned. You press a hotkey to broadcast left click (this is able to be done in game with the default UI) and a click broadcasts. To accomplish this with pure software you will find that you need to position the cursor on each window before attempting this due to moving the mouse out of teh windows bounds you will have a mouse on the outside edge of the window.

My thoughts? The movement to a predetermined position is very similar in my mind to KeyClones hotstrings. You press a button and the software translates it before it reaches the other machines. I can see how this would fall into the area some purists would prefer to stay away from, but I think it still falls well within the spirit of 1 physical action = 1 action per client... unfortunately my opinion doesn't weigh heavily when Blizzards the ones making the call, so I've avoided this sort of thing in my IS setup.