When you call pwm_init() to set up the PWM output, what's the return value? That should be the maximum frequency possible for those outputs.
I highly doubt it's possible to hit 1MHz and have such a small duty cycle as 1us. What are you using the output for that requires such fast timing?
Looking at pwm_set(), it appears that 1/1024 is the shortest duty cycle possible with the Rabbit 4000's PWM outputs.