I finally got around to playing with WordpressMU and I cannot for the life of me find an option that lets me create a new blog on a separate domain.
The best I can do is create a blog on a subdomain, then go through the DB and
change all occurrences of subdomain.example.com to example2.com in the wp_<blog-num>_options table
Then change the entry in the wp_blogs table.
Then create a new entry in the wp_sites entry for the second domain.
I figure there must be a more elegant way.
With plain old WP, I just add two lines to wp_config and create a symlink to the WP install and I'm done, which seems simpler to me.
With drupal, I just create a new directory and put a settings file in it with the address of the new DB.
The disadvantage of the drupal and the WP non-MU options is that when you upgrade the codebase, you have to run the DB upgrade script separately on all sites. So I suppose once WPMU is set up, it should be simpler, but I'm wondering if I'm just missing something here.