As noted in another message, I've just installed OpenSSH on my
machines.  After doing this install, I noticed that X11 forwarding no
longer worked for me either.  The solution (for me) was to change the
ForwardX11 option in /etc/ssh/ssh_config and /etc/ssh/sshd_config from
"no" to "yes".  If you don't have such and option in these files, it
may be necessary to add one since the binary may be compiled in such a
way to shut it off by default.  (This appears to be the case for the
ssh binary.)

Once you've made these changes, you'll need to restart sshd on
machines for which you've changed the sshd_config file.  (Your
config file may reside in a different location than mine.)

Kevin