PaulS:
You can't use an analog device, like an FSR, to generate an input that wakes the Arduino. A digital device, yes.
Very much to my surprise, this can be done and it works exceedingly well! I have a project with several tactile button switches and an FSR. It sleeps in power down mode; I wanted any button or the FSR to wake it. I treat the FSR identically to the buttons, I have the internal pullups enabled and use pin change interrupts to wake the µC. I use the same library for debouncing and sensing the FSR and the buttons. Actually, for this project I never use the FSR as an analog device, since I just want to know if it's "pressed" or not.
Going in to the project, I of course assumed that I would have to treat the FSR as an analog device. But in a moment of weakness (and temporary insanity), I thought I'd try just treating it the same as the switches, nothing to lose really. I was amazed when it worked perfectly. Maybe I got lucky, maybe the characteristics of the FSR I chose, the way it's being loaded, and the hysteresis provided by the AVR all just lined up perfectly, so YMMV, but it works just great for me.