On the mac, Word assumes a 72PPI screen. So, 100% should be correct on a 72PPI screen. That is, the display size will appear the same as if you put a sheet of paper of that size up in front of you. Unfortunately, until we get a resolution independent OS in 10.5, we're stuck with this rather arbitrary limitation.
On Windows, it is possible to specify a screen PPI manually. As such, it is possible to have 100% to be real life sized.
Adobe Reader and other Adobe appilications have the ability to override the screen PPI given by the OS, so 100% on Adobe Reader can be made to be real life size. This is independent of any settings of the OS. Firefox does this too.
I find the 72PPI limitation silly. You can imagine that if you were using an all Apple solution, then the OS should know what the proper PPI is and adjust accordingly. Of course, if you used third party screens, there is no way to tell what the actual PPI is. But c'mon Apple. You know the PPI for your own products. How hard is it to make 100% represent real life size?
I should add that I understand that parts of the OS is written in units of pixels. So things like raster images should display pixel for pixel. However, things like text should be fully scalable since fonts are vector quantities for ages now.
OK. I just realised I've hijacked your thread without answering your question. Frankly, I don't think there is a solution.
PM G5 Dual 2.0GHz, 2GB RAM
PB G4 1.67GHz, 1.5GB RAM
PM G5 Dual 2.0GHz, 2GB RAM
PB G4 1.67GHz, 1.5GB RAM