I guess the real question here is: what is the maximum CURRENT that can be safely passed through Zalman Fanmates?
Resistors work by reducing the voltage that passes through them. They create a voltage "drop" equal to what their resistance is times the current, which has been correctly stated by adam1234.
However, power is equal to the voltage times the current. In this case the Tornados are drawing 0.9 amps. Since I have my Fanmates turned almost all the way down, I'm going to estimate that they are allowing about 6 Volts to the fan. That gives a power dissipation of (6 V)(0.9 A)=5.4 W...below the "allowable wattage" of 6 W. To tell you the truth, I'm not even sure what Zalman means by that, since power is simply a derived value based on the product of voltage and current. Does it mean that the Fanmate can only dissipate 6 W, or does is mean that whatever it is controlling can only dissipate 6 W or less? If the latter is the case, the obviously there's nothing to worry about. If the former is the case, then I still don't see how this can cause the a problem, since the power dissipated by the Fanmate is the voltage DROP times the current:
12V - 6V = 6V drop
6V*0.9A = 5.4 W....