Menuwhere looks interesting since I have always run (3) 27" Displays as well. What I can't believe, is that someone never developed software that allows me to use a mouse in what Wacom terms "pen" mode! I still use a Wacom tablet with the old Wacom Mouse (no longer supported in new tablets) for most functions. The stylus is great for specific tasks like in Illustrator and Photoshop, but is a nightmare for navigation tasks - mouse is much faster and more accurate (for me). With Wacom I can "map" my Wacom mouse to an area on the tablet to span multiple monitors. Same thing could be done by defining an area on a desk with a 3rd party mouse, but nobody every wrote the software. Would be great for even 2 monitor setups - and people could run the mouse of their preference. Amazing that some high end mouse manufacturer never wrote that software as a selling point - I would buy it in a minute.
The hard part for using a mouse in pen mode is that the mouse would need a mapped, absolutely-tracked area to work in, rather than the relative tracking they normally use. Something like a Leap Motion hand tracker could probably do the job, but at that stage, you've got hand tracking, so the mouse is a bit redundant.
I used to have one of the old serial-connected A3 electrostatic surfaced Wacom tablets, flick a button and it would suck any paper you put onto it down onto the tablet like it was magnetic - darnedest thing. It let you use the pen and mouse at the same time, and have two cursors on screen, so you could grab things by multiple points.
The secret to making the stylus good for navigation is it really needs to be as close as possible to 1:1 for the size of your screen. That said, I personally believe the old serial tablets under classic MacOS did something undefinable differently (whether it's lag or what), because I used to use them all day to drive my machine with the pen, and that's never felt as comfortable under OSX.
