How to update WordPress from the latest trunk

I read somewhere that I can set a constant in the wp-config.php to update WordPress from the latest trunk version and not the stable version.
Stupid as I am, I can’t find the site anymore where it was written.

I know I could also update with SVN, but I thought the autoupdater was a smarter version.

http://wordpress.org/extend/plugins/wordpress-beta-tester/

Sets you up for the nightlies, if that is what you are looking for. I’ve got it running on my dev install @ cdn.rvoodoo.com. Works great there