It's been noted here and elsewhere that X11 in Leopard is different from Tiger. You don't have to (and in fact, shouldn't) launch X11 explicitly in Leopard. Just do an ssh -Y hostname to a remote machine in Terminal, and when you launch a remote X app, X11 automatically starts in Leopard. You'll see it in the Dock. What's not obvious is something that happened to me today:
I upgraded my home Mac from Tiger to Leopard, and everything went fine. It was with the "fresh" Archive and Install on my laptop where I had issues with X11. Basically, everything seemed normal, except X11 apps would not launch. Long story short, it was the Archive and Install from Tiger to Leopard that was the problem!
A clean install of Leopard will not install X11. However, if you're coming from Tiger and already had X, the old X11 app is not removed from your disk! So you'll see the presence of X11 in your Applications folder and be fooled into thinking you still have X11 installed. And in fact, it will start up in the Dock the way X11 is supposed to in Leopard when it's called from a Terminal command that needs it. It just won't actually work -- very confusing.
The solution is to insert your Leopard DVD (no need to boot from it) and install 10.5 X11 from the Optional Installs installer. Then everything will be fine. Or just do an Upgrade install at the outset and have no problems!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071217205635522