I'm not sure if it's possible with the n900's screen, not with any kind of real accuracy anyway. However, you might want to check out http://doc.qt.nokia.com/4.6/qtouchevent-touchpoint.html and try pressure() it to see if it works.

If this doesnt work, what you may be able to detect is how much of the screen is being pressed, for example a light touch of a finger tip compared to a jolly big prod with a thumb. Or the length of time an area is touched. However I'll be interested in your results to see if it works consistently.