Apparently, despite an avalanche of negative feedback with respect to security and privacy concerns, the Bush administration has decided to insist that all US passports issued as of 2006 be implanted with RFID chips.
RFID chips are little radio transmitters that can be detected without the passport being placed in direct contact with a detecting device. Other people, including other nefarious people, can detect the transmissions. Your transmissions. From one of your most important citizenship documents.
Supposedly the passports will provide some mitigation to the dangers of being intercepted. But I am not comforted - I am inclined to think these measures will be insufficient, and that in general this is a path down which we should not be travelling. I'm all for making passports unforgeable, but that's not what this is about. This is about tracking people, and, like most such efforts, is so enthusiastic about being able to track people in new and exciting ways that it turns a blind eye to any of the problems such tracking happens to cause.
In the meantime, to avoid having a RFID-"enabled" passport, I think you can renew your current passport now, even if it hasn't expired. (I'd renewed last time when I still had a year left since it was a year I wasn't travelling abroad and didn't need it for a while. It's getting harder and harder for me to find an occasion when this is true...) Also, if you're running out of space in your passport, you can have pages added. The easiest way to do this, in my experience, is to go to a US consulate abroad and they can do it for you while you wait.
Interestingly, getting pages added is a remarkably straightforward and paper-based endeavor. They have a little booklet that they tape into the center, and then they use an engraver to emboss an official seal on it indicating that it is a valid extension to the passport of this particular number. No silicon chips, no power cords, just paper, tape, ink, and an engraver that pinches paper into a design.
Edit 10/27: In case you are saying to yourself, well, I don't travel abroad and don't need a passport, bear in mind your driver's license will have one of these things by 2008. Read more about it, and be very alarmed.