edit: wow, i got beat to the punch... : )
if i remember correctly, here is the basic history of it:
Castling and moving a pawn 2 on its first move came into the rules in European chess around the same time, and limitations were put on both. You probably already know you cannot castle out of check, or through check--the reasoning behind not castling through check was that even though he moved through several squares, the king only moves one square at a time, and thus would put himself in check along the way... but the pawn rule is just as easy and makes as much sense. For the one turn that you moved a pawn two, the pawns that could normally take it had it moved just one can take it IN PASSING (en passant), essentially, the pawn takes time to move, and can be killed on the way--however, if they do not en passant THAT TURN, he will have already arrived at his destination, and cannot be taken with en passant as he is no longer passing, but rather has already passed. You'd be amazed how many people have never even heard of en passant, and would challenge you if you tried it with them... most of them don't know a whole lot about chess, but a handfull of them do.