Most Phenom II cpus will put load on a single core if it can because it saves power, and therefor lowers the amount of heat generated, mine will do this to when i run certain programs.
I really doubt it....mostly there's a single threaded application running in the background.
If there are multiple idle task threads to be executed, it makes more sense for the OS to run them on 4 separate cores @800MHz, rather than on a single core @3.2GHz. This is because as clock speeds increase, the power dissipated by the core increases following a squarish kind of law, giving off more heat.
Same reason why server chips have more number of cores that are clocked slower than desktop chips....its more energy efficient:)
Phenom II chips CANNOT switch off individual cores. The best they can do is idle them at 800MHz.