There is a small plastic piece inside the metal hinge that swivels back and forth due to gravity and the decelleration from appying the brakes.  Basically, it causes the seat to lock in the upright position when the car is rapidly stopping (so that the weight doesn't hurt you in a crash), while it SHOULD be unlocked all other times.  Your's is sticking.  Mine did the same thing.  One-time fix is to tap on the hinge to get the plastic piece to fall back down into it's normal position and then you can fold the seat.  Longer term fix for me was to spray some graphite power into the hinge, all around the white plastic piece. 

chesh is right, samething happened to me, a little lube and in my case, a small screwdriver, it was fixed.

Denny

I don't remember if both sides have this locking mechanism or just one side.  I'm guessing both, as it is a safety issue.  These seats are very flexible, so you can tell pretty easily which side has the sticking lock by pulling the seat forward with moderate force and you'll see it twist a little because one hinge is working and the other not.  Of course, there is always the remote possibility that both of yours are stuck.  I should add a couple of things:

At hardware stores, you should be able to find the graphite powder in a small tube that had a small, pointed end with a screw off cap.  You can squeeze these containers to "squirt" powder into tight places - exactly what you are looking for.

Also, after I sprayed graphite powder in there, use a couple of small screwdrivers or something similar (one going in from the front of the hinge, the other from the rear) to push the lock back and forth, to work the graphite into areas not reached by the squirt.  Then I repeated the squirting and working the lock back and forth a couple more times for good measure.  Haven't had the problem since.

Note, if you take the seat out of the car to work on it, if you are testing it in any position other than  the natural reclined position they are in when installed in the car, you could think the lock was stuck when in fact gravity was just making the lock do what it's supposed to do.  In the normal inclined position of the seat, gravity should pull the lock away from the little tang in the hinge and allow the seat to fold.

Hope that helps!