X stopped working correctly when I upgraded to Tiger, the primary culprit seems to be a new version of ssh (see ssh man page on -X and -Y flags). I did two things to get things to work again. Added trusted X11 forwarding, so I now have:
ForwardX11 yes
ForwardX11Trusted yes
in...
I ran into this too... you'll also see "Xlib: extension "GLX" missing on display" errors as a manifestation of this problem.
The issue is that the behavior of SSH changed in Tiger. Apparently now some operations aren't allowed (for security reasons?) with traditional X11 forwarding.
To...