If it's the same thing that is in the new BMW's, it's called a charcol air filter. It is supposed to stop the leakage of gas vapors. No one from BMW forums can figure out the exact use for it either. One thing is certain though. If this is what's in the new Stangs, then 1. it is messy to take it (has to be cut up), 2. can not be put back in, 3. is only required in the states, 4. increases power a little when removing it ( the new M 5's gained 20-30 hp).
Anyone else out there know if this is what is in the Stang?