Illuvatar (or shall i call you Eru

) your theories are intriguing. I've never thought about it before but it makes perfect sense that the Hat would know that Voldy was Slytherin's heir. Up until now I thought that dumbledore kept close tabs on Tom Riddle because he was adept in leglimency and so knew what Tom was up to. But perhaps it was because the hat told him about Tom.
If I had a hat with a 1000 years worth of wisdom, that had been given brains of the 4 greatest wizards of that time, that's what I'd be taking advice from!! Maybe because the hat has seen inside of Tom Riddle's head, it knows what Voldy is likely to do, not to mention all of the dark wizards that went to hogwarts in the past. The hat could be a master tactician for the Order.