After upgrading to Mac OS X 10.4 (Tiger) I found that I could no longer
display X Window applications running on a remote computer. The error
message was:
X11TransSocketINETConnect: Can't connect: errno = 111

That is, after bringing up X11 on your Mac you do
1) xhost +IP_of_remote_computer
2) ssh remote_computer
3) setenv DISPLAY IP_of_my_Macintosh:0
4) xterm

Alternately (as I recently learned)
1) ssh -X remote_computer
2) xterm

or same as above but with -Y

Other accounts on my Mac didn't have this problem. I had changed the
password on my account on the Mac but changing the password on another
account didn't reproduce this problem. After some trial and error I
found that tossing out the file /Users/my_account/Library/
Preferences/com.apple.x11.plist
solved the problem (the system will automatically generate a new and
functional file).

Donald