Yes, I think you should be able to connect the electret directly to a pic pin, thru' a capacitor. Just remember that the signal would rarely be a ttl-level swing, so chose an input pin that can be configured as an opamp, and count every time the signal goes higher than your refernce. Alternately, configure an input as an ADC, and again decide on a threshold, to detect the signal. With this, you should be able to measure frequencies of a signal with an amplitude of a few 100s of mV. I would also use a clamping diode, just in case the mike gives a high pulse.
Regards,
Anand