Jeff, when 3-way switches are replaced with X-10 switches/dimmers, what happens is that all of the switching/dimming is done at one switch's location by the 'master', and the companion 'slave' switch is basically a wired remote for the master, and replaces the second 3-way.

X-10 was designed from the beginning to be used with existing wiring, for 'universal' compatibility. With a few exceptions, either X-10 unit may be installed in either original location; a few X-10 models require the master in one particular box.

Larry
www.fineelectricco.com