Check with your local recycling program, but I bet if you're in the US, your local program won't take car seats. The recycling center in our town in Germany did recycle car seats. When you dispose of the seat (recycling or otherwise), be sure remove the straps, cover, etc (anything that will come off) and dispose of those separately. Then, either take a sledge hammer to the shell or write on it with permanent marker something like, "Broken! Do not use!" or some other dire warning. You'd be amazed at the lengths people will go through to use an old seat (like using bungie cords for straps!). If you put it out with your trash, try to disguise it, as well by putting it in a black plastic bag with other stuff.
Ulrike, mom to:
Roman (3/98), Evalina (3/00), Nadia (3/03), and Kira (11/07)