[CentOS] ssh X11 forwarding not working

I'm having problems with ssh.com and X11 forwarding

ssh: SSH Secure Shell 3.2.0 (non-commercial version) on i686-pc-linux-gnu

I am in the process of testing some of my machines with CentOS 5 x86/x86_64 and have run into a bit of a snag. X11 Forwarding is not working.

I've tried all combinations of (+/-)[Xx] and it doesn't work, seemingly it doesn't want to set the environment variable properly and therefore doesn't know where to send the X requests.

Any hints? I've checked the ssh config files and they seem to be identical to the config files on our existing SuSE 10.0 machines which are working. This is troubling indeed.