Can i run 9 speed with a 10 speed Derailleur?

[Red.]

Well as it says above. I have a 9 speed shifter and if going to buy a new "SRAM X-9 Rear Derailleur 10sp 2011" and i was just seeing if i have to run it at 10 speed or can i do 9 speed gearing??

 

[muskimo]

your shifter will only be able to select 9 gears no matter what your rear end is, though in saying that you can go backwards, ie; 9speed down to 5 or 3 or 6 etc, as you just lock out the derailleur. you cant add a click to your shifter. (to my knowledge) just buy a 10speed shifter, if your spending the cash do it right

 

[No Skid Marks]

Any numbered shifter will work with the same numbered cassette. Now chains are getting thiner again, I'd just check that the big Ss haven't made the mech cages narrow so as not to fit a fatter chain. But this is doubtful, if even possible at all.
So in short, yes it'll fork.