XDMCP only seems to be working for broadcast even when configuring /etc/X11/xdm/Xaccess. I remove everything and just added one single line: * Broadcasts work fine with the default setting (* CHOOSER BROADCAST), but XDMCP queries (and indirect queries) don't. This holds true for kdm, too (which is based on xdm). However, on the same machine gdm works (after editing /etc/X11/gdm/gdm.conf and the useful tip in a newsgroup to add gdm to /etc/hosts.allow - tried this for xdm and kdm too, no success). There isn't a log entry produced either (only gdm did when not being added to hosts.allow).
Red HAt 7.0 does set DisplayManager.requestPort: 0 in /etc/X11/xdm/xdm-config for security reasons. Commenting out this line makes xdm/kdm work. Closing bug.