  Mike (Murphy), Well I had a nice reply to this, then I switched pages in my browser and it blew it away, so here's the take from an article on mlb.com The Infield Fly Rule was instituted to protect the offense from a possibly "unfair" double play. If there were runners on first and second (or bases loaded) and less than 2 out, runners would normally hold their base on a pop-up to the infield since the ball would, in all likelihood, be caught. Without the Infield Fly Rule, an infielder could drop an easy pop-up (or let it drop untouched) and turn a double play, since the runners would then be forced. The rules makers didn't think this was fair and instituted this rule. The Infield Fly Rule is only in effect when runners are on first and second (or bases loaded) and less than 2 out.
It states that the batter is automatically out if he hits a fly ball "which can be caught by an infielder with ordinary effort. " The umpires loudly announce and signal, "Infield Fly, batter is out! " so that the runners are alerted that, even if the ball is dropped, they do not have to leave their bases. 
