It sounds like either the brake light switch is faulty (cheap and quick), or the shifter ($$$) is bad. The good news is that there's a simple and quick way to figure out which part is the problem. Get in the car and close the doors and windows, so it's completely quiet. With the ignition switch on, step on the brake pedal and listen for a "click" from the locking solenoid in the shifter. You can also feel the solenoid activating by placing your hand over the shift gate. If the solenoid can be heard or felt when applying the brake, it means there's a mechanical fault in the shifter. If the solenoid doesn't work, it ususally indicates a bad brake light switch.

If it turns out to be a bad shifter, you don't have many options. Since there's no emergency release on the shifter, the only way to get it out of Park is to disconnect the shift linkage. I've had some success though, "tapping" the top of the shift lever while pulling back on it. Sometimes the solenoid will move just enough to allow the shifter to move out of Park. I hope it's the switch though. At least it can be fixed on the spot.