Seamus,
Break the problem down yourself. What period are you measuring 10msec or 2.5msec ? Do you need to average the data ? THEN look at the timer hardware and see exactly what it can do. Use my guidelines about gated timers yesterday perhaps, to yield you two 16 bit numbers that represent the precise ratio.

Hang on, better idea comes to mind: use an op-amp as a buffer to the accelerometer and a capacitor as a filter and extract the PWM signal as an analogue level using the ADC on the ADuc 512 - MUCH simpler still, if you don't need 16 bit precision, and from your description you don't. The FPGA is complete overkill I reckon. Cute, but overkill. The best solution is the one that works, not the one you like the idea of.....

Steve