Ukimix: Ukimix: Actually, there is a variant called Nuclear chess aka Dead Square Chess that has identical rules to your proposed variant. Here is a link:
http://www.chessvariants.org/difftaking.dir/deadsquare.html

OLTI27: This does sound like an interesting variant. There is another variant called Cursed Chess, which is somewhat similar, although not exactly the same. When a piece is captured it curses the square it is on so that enemy pieces are unable to cross it once the capturing piece leaves. Friendly pieces can still use the square though. The full description is here:
http://www.chessvariants.org/difftaking.dir/cursed.html

Here's an interesting variant by the inventor of Racing Kings (Vernon Parton). The rules are described here:
http://www.chessvariants.org/winning.dir/neutral-king.html
Basically, neither player has a king of their own, but there is a neutral king that may be moved by either player. The object of the game is to checkmate the Neutral King and prevent your opponent from doing so. I don't think there are ANY websites that have it! (Although there is a java applet at pathguy.com where you can play against a computer. But there's nowhere you can play against real opponents.) Brainking could have a world first!