I wanted to get KDM to display a list of XDMCP hosts willing to export 
their displays.  A very simple GUI is presented to the user listing 
hosts identified by a broadcast for willing hosts.  There is practically 
NO documentation explaining how to get this done with KDM, but tons of 
people asking the same question.  Anyway, the solution was to edit the 
/etc/X11/xdm/Xservers file.  There should be a call to X next to a 
0:localhost display indication.  You need to add the option -indirect 
<xdmcp.host.to.do.your.broadcast>.  Make sure you add this option FIRST 
to ensure that X doesn't treat the your parameter as a command-line 
argument.  I've been battling with Mandrake 8.2 on this one for about 3 
weeks, and I can't believe that a solution was nowhere to be found :(

Also, if you installed KDM from RPMs as I did, you could have a path 
problem.  KDE2 installs its executables in /usr/bin while KDE3 uses its 
own directory.  You want to make sure that your installation is calling 
the correct kdm version.  The solution I used was to edit 
/etc/X11/prefdm and make sure that the KDE3 executables directory is 
prepended to the PATH variable in that file.

If you need more clarification, feel free to contact me.  I just figured 
this out last night, so I haven't yet reported the solution to any other 
forums.