WOW! it is because of threads like this i love this forum, taking part of different peoples often brilliant ideas, it feels great.
However i'll put my two cents in here to.
Force or pressure? i think both will need a solid foundation. Force sensors, i think you will need to have one in each corner of every tile, expensive and i actually dont think they will stand up against the elements of nature, rain, snow etc. Not the ones linked to anyway.
I made a 3D-"sketch" of the pressure idea wich was a very brilliant idea.tile3
, on Flickr
As you see there r some things you have to consider, fixate the tubing, dirt may fall down between the tiles, the sensor and hose clamp needs to fit somewere, the tiles might be swampy to walk on and so on but i am certain that it is possible.
Another idea was a capacitive sensor: well... i dont think so, dirt rain and wear will make it difficult.
And there was the accelerometer idea, wich is very very good, it might need some analyzis of materials, mountings, soil-damping and so on it was so interesting it made me think of a similar idea but with a michrophone instead.tilemic
, on Flickr
Think of it, when you put a glass to a wall and your ear to the bottom of it, it amplifies the smallest taps to almost painfully loud noise. I thought you could make it similar with the tiles but maybe with a can or so. In the bottom of the can you put a small michrophone. It will be easy to make a hole in the soil so the can and mic will fit. It will take some testing first and probarly lots of software debugging, but hey, thats half the fun.
It will be like AC-coupled though, people needs to move to make it light and rain will probarly trigger it to. But maybe that would be cool?
I did not mean to step on anyones toes here, this is just my thoughts about it.
And sorry about flickr spam but dont know any other way to put in pictures here.