I'd never heard of this tool before. I found it in google webmaster tools. It sounds like a good feature, any one used it before? Would you still use a 301 if you used this? http://googlewebmastercentral.blogspot. … omain.html

the answers in the blog

If I use this feature, should I still use a 301 redirect on my site?
You don't need to use it for Googlebot, but you should still use the 301 redirect, if it's available. This will help visitors and other search engines. Of course, make sure that you point to the same URL with the preferred domain feature and the 301 redirect.

this only works for Google