The terrorists use of this technique is crude, and it has limited application (can't be used in paved roads, takes longer to install the bomb, and the fuze is a more complex mechanism, meaning there are more components that can fail.) The Iraqi terrorists have been using pressure activated bombs, of various designs, for over a year. But the results have not been encouraging (for the bombers). Continued work on a device like this is more a sign of the desperation the terrorists are facing, than any major advance in their technology. The electronic jammers have been very successful, and have made it possible for military and supply vehicles to travel over a larger number of routes. The most successful alternative to the jammers is using a wire connection to set off the bomb. Again, this takes more time, and exposure of the bomb planting crew, to carry out. As a result, "wired" roadside bombs tend to only show up in Sunni Arab areas that are both very pro-terrorist, and not policed. That's a combination that is becoming more rare with each passing month.
Iraqi terrorist bomb makers are trying to learn something from the designers of naval mines. The terrorists are using pressure mines to get around the electronic jamming that most convoys and military vehicles now carry. These jammers block the signal (from cell phones, garage door openers, Etc.) used to set off the bomb just when the target vehicle passes next, or over, it. A pressure fuze, buried under the road, would only close the circuit, and detonate the bomb, if a certain amount of pressure was exerted. Thus the pressure fuze would only go off if a heavier military (or supply) vehicle rolled over it. This could be combined with a wireless device that would arm the bomb. The wireless signal would have to be sent before the target vehicle got close enough for the jammers to block the signal, but this would prevent the bomb from going off for just any truck that was heavy enough.