Originally Posted by cyccommute
Absolutely incorrect. You can run just about any derailer on just about any cassette as long as the brands match. I've run Shimano 600 (clearly pre-9 speed) on 9 speed cassettes using 105 9 speed STI shifters without issue. I've run 7 speed XT on 9 speed cassettes, also. It's just not an issue. The derailer doesn't really care. It's a robot. The shifter sends the command and the derailer does what it's told.
The pull ratio, the amount the end of the derailleur lever moves per mm of cable motion, changed when 9-speeds came out. Previous to that the DuraAce had one ratio and all the other Shimano derailleurs had another. When they changed it they used the DuraAce ratio.
Sheldon Brown says that you can use the earlier derailleurs with the newer 9 or 10 speed STI levers but I have tried it and it does not work reliably. I tried his idea of changing which side of the lock screw to attach the cable and it made no descernable difference with my bikes.