Will a 9 speed front derailleur work properly with a 10 speed sprocket? I am pretty near done my current bike build project, just missing a front derailleur. I can't think of why front derailleurs would need to be compatible with different numbers of speeds, given that the width of the sprocket will be about the same for an 9 or 10 speed cassette.

So will it work? I bet it will.

edit, FYI: I have this installed and it has been working fine with a front derailleur marked as 9-speed for about 6 months.
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).