Sending a signal back in time

If only you could. Of course, if it were possible, why hasn't your future self sent you a message to prevent some stupid thing you've done (in last week's The Office, Jim, using pilfered stationary, sent Dwight a fax from "Dwight's future self" alerting him to a potential catastrophe; hilarity ensued).

Via Slashdot (where else do you get good geek news about reverse causality?).