Will an 8 speed chain, cassette, and shifters operate with a 9/10 derailleur?
it 'should work, although I would check the cage width.
there may be some subtleties in how well it works though.
edit: a quick check on Tristans site agrees with me
http://www.wheelworks.co.nz/drivetrain.php?ST=S1&FD=S1&RD=S4&CN=S1&CS=S1&CR=S1
Agreed. Everything should work fine, but you will most likely end up with more chain rub on the inside of both front and rear der. cages in certain gears than if you used a full 8sp set up.