NY issues a special one for bikes. Looks the same as the car one, but it has a big "M" on it. The toll pricing structure is different also. Less for bikes.

http://www.panynj.gov/bridges-tunnels/tolls.html

http://www.ny.com/frame?url=http://w...me/travel.html

 

I don't know about anywhere else, prob all the same, butt in Florida you register the vehicle to the EZ, now whether you have it or not, it reads your tag and charges that account.
So it really doesnt matter where you put it,
I carry mine in a windsheld bag. I have put it inside the lid on hardsaddle bags and it worked also.

Now I was told If you go through the toll with out your EZ when your tag is run it charges that account, IF your tag is registered on several EZ's then it charges ALL of them.
So if you do this beware of the chance of being overcharged