Well, yes this would allow individual X applications to display on your Macintosh X server. Although it's not really "piping" but "tunnelling X". However, what if you want to connect to the remote X server via XDMCP such that you'll get a full-screen login (similar to what you would get if you sat the system console)? Well, turns out you can do that too.
Ideally, you would do:
sudo X -query server_name_or_IP_address
however when I tried this to connect to a Linux box, no dice. I got a "XDMCP fatal error: Manager unwilling Host unwilling" even though I can connect just fine from a Win2k PC with Cygwin. But you can do this:
sudo X -indirect server_name_or_IP_address -terminate
 You probably want to use the "-terminate" flag to prevent cycling when you logout (i.e. another XDMCP Host Window popping up)[/edit]
After a moment, the familiar XDMCP Host Window pops up. You can add the IP address into the Host window and click "Add". It then shows up in the list of hosts, and even though it reports (for me anyway) "Display not authrorized to connect" alongside the remote X server name, when you clik on it and hit "Accept", a moment later you've got your remote desktop login window.
One caveat - my tests of this don't allow custom sizes (i.e. as in Linux where you could say "X -screen 0 800x600+0+0") so it's full-screen. Hence the friendly OS X Dock will get in the way. So you might want to shift it, or your KDE or Gnome taskbar, to the sides so they don't get in each others' way.
Tested this on OS X 10.4.2, remote system SuSE Linux 9.3 Professional.