7 speed front Derailleur on 9 speed crankset I have 9 speed crankset (44-32-22 FC M542), and matching RapidFire 9x3 speed shifters (FL M590).

Can I use 7 speed front derailleur FD M550 in that combination?
9-speed chain is slightly narrower, so a "9-speed" front derailleur might have plates slightly closer together, but that's not going to make a lot of difference:
 http://sheldonbrown.com/speeds.html#derailers

So if the FD M550 worked with a 7-speed 44-32-22 crankset, it will work with the 9-speed equivalent, but a new 9-speed might shift very slightly better.