I am having problem with this. I just installed the latest version with WP 3.1.1
1) I have swapped the A name to my dedicated IP address; and
2) Typed in the custom domain but when I click add, the domain name disappeared.
3) I have recently upgraded Multi DB to latest version (by swapping the db.php file, as advised) before installing domain mapping.
4) I have checked the db-config file and there is a line for adding domain mapping global table
5) I checked the list of tables in PHPMyAdmin and I don't see a domain mapping table crreated under the global database
6) when I type in the custom domain name it treated as if the name does not exist and redirected me to the signup page with the custom domain name prepolupated.
7. When I type in my IP address, it no longer point to my main page and redirected me to the signup page with my IP address pre-populated.