The domain mapping plugin works well except for one issue.
My Wordpress 3 MU install is using subdomains for each additional website.
So I have my domain.tdl.
I found that if I typed in a subdomain that didn't exist, (nositehere.domain.tdl), it would redirect me to http://domain.tdl/wp-signup.php?new=nositehere which is the signup for with "nositehere" already prepopulated in the Username field. Pretty neat.
However, after installing domain mapping, when I try and visit my IP address (http://173.xxx.xxx.xxx/), it redirects me to http://domain.tdl/wp-signup.php?new=173.xxx.xxx.xxx with "173.xxx.xxx.xxx" already prepopulated in the Username field!
http://173.xxx.xxx.xxx/ should redirect to domain.tdl...
not redirect to http://domain.tdl/wp-signup.php?new=173.xxx.xxx.xxx