I have the exact same problem on Mac OS X Leopard 10.5.8., on my Mac Pro Desktop. (I am not affiliated with egadsby) I am using a HP Color LaserJet CM2320fxi MFP. This printer supports duplex printing - I print duplex all the time - but I have to always activate it.
Greg, the "Duplex Printing - Mac OS X" solution does not work. I have already tried it five times. The two-sided printing option does not stick. I can follow those instructions, create a new preset, etc. But - when I activate the preset (or if I use it as the default preset), the two-sided printing option simply deactivates itself, automatically. There does not seem to be a way for the two-sided printing option to stick with any preset.
Frustrating... I am stuck with having to specify duplex printing each time I print. I always want to print duplex, so this is slowing me down.