I've been able to successfully use Apple's X11 Beta 3 for XDMCP connections to other hosts using Mac OS X 10.2.4. I start a terminal session, and define a display variable (e.g., 'setenv DISPLAY :0.0'). Note if you do not do this first, you will get the behavior you described of having the Xserver start and then appear to do nothing. Then, I execute a XDMCP query X session start (e.g., 'Xquartz -query ip-addr'). This starts the session, and you will see the messages you described appear. You will also get a new docked icon whose name is the same as the Xserver you started (e.g., 'Xquartz'). If you select this item with the mouse, and then move you mouse off onto the normal screen and wait, you should get your full-screen Xsession from the host at ip-addr to appear. This first screen takes a while to come up, but subsequent screens are very quick.