You know some physicists have a theory that time travel back in time will be restricted to the time when a functional time machine was invented. So at the very instant when a physicist turns on the worlds first time machine massages or possibly people from the future will start to arrive. I'm not sure about the reasoning or the physics behind this prediction but it seemed interesting when I saw it. I think it was on the science channel.
And for the record, I don't think time travel will be achieved by using old Leave it to Beaver shows.