Will a 9 speed front derailleur work properly with a 10 speed sprocket?
As long as your front derailleur matches the number of gears on your front chain ring (2 or 3) you should be okay. 

That being said, you may run into issues with different manufacturers or groupsets working well together. Are you using the same make / manufacturer in the front and rear? (e.g., I know Shimano makes both a 9 and 10 speed version of the 105 groupset and I am fairly sure the front derailleur is the same).