Will Windows 7's multi-touch work on every touch screen?
Edit: Ok, do you think some of these drivers would work backwards compatible? If not, how would I recognize if it supports it (if it's not of the box)?
No. the screen will have to support multi touch. You can't just use a normal single touch screen and expect to automatically get multi touch features from it.
Beyond that, there will need to be appropriate windows drivers for the specific screen. I would assume most manufactures will provide windows drivers, it would be pretty strange if they didn't.
[Edit: in response to your edit. It's possible some manufactures will make their drivers backwards compatible. We have a touch screen here (that actually wasn't sold as multi touch) but it supports a kind of limited multi touch (only 2 touches allowed at any one time). Because there isn't any native windows support for this kind of thing in windows XP though you have to access the multi touch functionality by coding directly against a com library they provide, so any code written for it is specific to that manufactures screens that they support that particular library on. With windows 7, MS have provided a wrapper layer so you can write the code once to handle the multi touch functionality and provided the drivers are in place it won't matter what screen is being used. If your screen doesn't say it's multi touch compatible, it's not. The hardware is different to normal single touch screens]