Why doesn't this kind of device exist for a long time already?
Since I have a MacBook PRO which allows using multitouch gestures on the trackpad (especially scroll and zoom) I miss these a lot when I work on a desktop Mac.
The "photo" above is of course a very amateurish work of Photoshop editing my external keyboard and and the MacBook's trackpad together.
However, should I get access to a broken MacBook body with the trackpad still functional before Apple or someone else makes a real product like this, I'd probably try to create one myself.
As the internal trackpad is a regular USB device (only connected internally), all I'd have to do would be connecting it to a normal USB cable, cutting the Trackpad plus the needed frame material from the MB(P) body, and putting everything together in a decent housing, probably made from a thick sheet of aluminium. I guess I'll be better at doing that in the workshop than in photoshopâ€¦
Donations of broken MBP cover plates are welcome - pointers to external trackpad products that might already exist as well, of course! But remember, it's the multi-touch I look for, not just an external trackpad.
[Update: just saw this product - altough it is for PC only and looks ugly to me, it is a step into the right direction. Still, I guess Apple's rumoured multi-touch mouse is more likely to provide what I am looking for]
[Update2: Indeed, it looks like Apple just released (kind of) what I was looking for: The Magic Mouse.]
[Update3 - August 2010: The Magic Mouse was a first step, but apparently they have really listened (to me? ;-)) and thus created the Magic Trackpad. I already got one and yes, it's exactly that what I wanted]