Hi,
I'd probably use a LDO regulator for the PIC and run the motor directly off the battery. You can always limit the dutycycle of the PWM to ~75% so that the motor never sees more than 4.5V average if that's critical. Running the motor directly off the battery would allow the motor to operate closer to its top speed even when the battery voltage has dropped a bit, (but then you obviously need to go above 75% dutycycle). If you use a 3.3V regulator for Vdd and as reference for the ADC you could monitor the battery voltage thru a voltage divider and use the result to compensate the PWM dutycycle as the battery voltage drops.

If you really don't want to use a regulator I guess you could use a diode or two but that would make monitoring the battery voltage a lot trickier.

/Henrik.