This plugin works, I haven't found anything that needs further development except the documentation.
Another plugin, written by the group I've heard is running the 2nd largest multi-site installation (#1 is wordpress.com), is http://premium.wpmudev.org/project/domain-mapping/
My Domain Mapping Plugin Instructions
I haven't gone through the code to see if the "plugin-less solution" covers everything; if you want to try that, you'll have to do a lot of testing to make sure Everything works for your site. Remember how much that testing is more work than using a plugin...