If you're a lot better geared than the tank, then it is your problem (if not actually your fault) -- it's better that you ease off the DPS a little than the mob be running off to hit you. Generallly though, taunt stacking can prevent this being a problem except early in the fight.
I have a 55 Sentinel and a 55 Merc dps... So I know what it's like from the DPS's point of view with regard to pulling and aggro management. I also have a 55 Assassin tank and a 55 Vanguard tank, so I know what it's like to try to hold aggro from DPS.
If a dps is a lot better geared than the tank, the tank WILL have to work a little harder to hold aggro, but if he's a good tank, he still won't consistently lose, and even when he does, IF he understands how the threat mechanics works in this game, there should NEVER be a time when a tank loses aggro, to the same dps, and doesn't have a taunt up to grab it back.
Good tanks can hold aggro off of geared dps... bad tanks can't hold aggro period. (I love watching a tank blow a taunt to START a fight... or hit their Taunt and AoE taunt back to back... it's just bad tanking.)