Geez, I can't imagine a worse setup than running off the headphone jack -- the impedance is wrong for hitting a line input, and the iPod's output amp is really easy to drive into clipping. I use iPods for stage work and I always run off the dock connector (using a PocketDock). I use monitor- and FOH-style boards, and have never had a problem getting enough levels to bring iPods "into the mix."
(I've had a couple of outdoor events where the gear was real crap -- ancient Peaveys begging to die -- and the input stage was dirty; for them I just picked up a 120VAC-to-12VDC converter and used a Belkin AutoKit's preamp to boost the signal ;-)
Me, I would want to use the dock's signal. If you've got your own little board or can borrow one (like a little Mackie 4- or 8-channel), bring it to use as a preamp -- iPod to Mackie line in, tie it to Aux bus, crank the Aux, and feed that to the house mixer).