Some disc brakes are designed with a cable pull requirement that matches STI levers.
Most disc brakes require more cable pull and need a Travel Agent or similar device to work with STI levers.
I'll expand on Retro Grouches answer a bit, he's essentially correct but I'll clarify.
Most mechanical brakes are made to match the same pull as V-Brakes, in which case STI's will not work. Avid as one example, makes BB7's in both Road and Mountain configurations, in which case the Road version indeed works with STI. BB7's are among the best going out there for mechanicals anyway