I had a tough time getting the alt / option key to work as the Meta (M-x) key in emacs. The escape key does it, but I wanted the key to be where I was most familiar. I'm on a new AlBook with Panther (10.3) and the Apple X11.app installed. Older postings here and elsewhere show how to remap using xmodmap. This works, but I think there are some particulars specific to Panther and Apple's X11.app that aren't reflected in the previous postings. Here are the details. There are just two steps to get this done.
First, put your remapping commands into an .Xmodmap file in your home directory ($HOME/.Xmodmap). With the help of the X11 program xev, I wrote a file that did what I wanted it to.
clear Mod1
keycode 66 = Meta_L
add Mod1 = Meta_L
It tested perfectly in emacs after running
xmodmap ~/.Xmodmap
You can see what your mod (modifier) keys are mapped to using
xmodmap -p
Second, open up the X11 preferences from the menu on the top bar. On the "Input" tab, uncheck "Follow system keyboard layout" to allow your keymap changes to overlay on the default X keymap. Then restart X, fire up emacs, and your alt / option key, or others you remapped, should work. Here's a bit more background on what I've discovered:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040425231058862