definitely worth changing at least once a year especially mikeee since you got a newborn on the way.

i went to the dealer today for some service and I asked why my AC smells musty. the service advisor told me it is due to built up moisture in the vents. The best way to avoid the problem is to turn off the air 5 mins prior to turning off the vehicle, and switching to the dry, outdoor air, until the car turns off, in order to dry out the vents. He also reccomended a product called Ozium automotive. spray under the feet area by the pedals, and by the kick panels of the passenger side with recirculate on, and to do the same thing with the intake vents outside of the car by the window wipers. it is a little strong at first of a deodorizer, but it definately tones down, and gives you that nice AC fragrance, instead of that nasty AC ODOR!

Hehe...yeah I think I need to change it every year now.

Here is the thread for the charcoal filter sale by Chad.