Well, if people stop mining, the difficulty will fall. If the difficulty falls enough, bitcoin will be cheaper to acquire and thus be worth less than their current estimated $4 of worth. Additionally, the release of the 7xxx series from AMD will probably speed up hashing. So, it's possible that if people stop buying bitcoins we could go to less than a dollar, yes. It seems kind of unlikely right now, though.
Wrong! Why does everyone always looks at "miners" as the solution or cause for anything?!
Difficulty goes down if miners stop, it's true, but also goes back up if they restart. So there's nothing to win or lose there.
The aim of difficulty is to keep coin generation at a steady pace, not to "open the bitcoin tap".
