Check out the pwm_meas module and the xtend_rssi module, and the the trigger_ext module for examples. I am not sure weather the xtend_rssi/pwm_meas modules are compiling right now, and don't have time to check or fix it, sorry, but feel free to report any problems anyways.
I used the xtend_rssi/pwm_meas modules to measure RSSI on an XTend radio modem, which basically has a pwm signal ranging from 0 - 95% duty cycle (I forget the period).
I am not exactly sure what you are trying to measure, but it might be fast to get some pulse measurement going by creating a new module like the xtend_rssi one, using pwm_meas underneath.
Hope that helps.