If everyone agrees to turn off their miners right before a difficulty change.. they could fake a lower network hash rate!
In fact I believe this is what has been happening with a few big players..the network hash rate always seems to drop right before a change and this makes mining [temporarily] more profitable.
I don't think this would benefit someone with a small amount of hash power, but a big player like myself might benefit from unplugging a couple hours before the change. i'll have to simulate this and see if its worth it.
brb k
edit: actually come to think of it, any gains from this technique should be wiped out by a counter affect from the following difficulty change... UNLESS it is pushed down EVERY time. There would be an accumulation of network hash power that is always hidden from the difficulty changes... could this be why there is an "up and down" trend consistent with the time of the changes?
I could see how this could cause a problem for the network if everyone decided to manipulate it this way.. We would basically be producing bitcoins at a faster rate than intended by the system. I guess the only side affect would be that the value of the bitcoin itself would go down (who cares, right?)