Detecting foot steps
Taking Nick's idea a step further, how about 2 plates with a slightly compressible dielectric between them. Then you've got a pressure controlled variable capacitor. Build a simple RC oscillator and measure the frequency of the output. 

Drive the whole thing with a microcontroller that calibrates itself when it's first turned on so it self-adjusts for the compression of the foam over time. 

This would be similar to your foam pressure sensor. You could probably use the same self-calibrating approach with that as well. At startup, you'd measure the resistance of each panel and make that your unloaded value. A significant drop in resistance tells you somebody's standing on the panel.