Description of problem: gdm used to be built with tcpwrappers on previous RHEL releases, although the BuildRequires was missing there as well. The tcpwrappers package just happened to appear in the buildroot.
I believe it's nice to be able to limit XDMCP connections using hosts.{allow,deny}.
Version-Release number of selected component (if applicable): gdm-2.16.0-30.el5
Additional info: If you're going to fix this, don't forget that bug 181302 applies here as well.