The Dragonborn Metallic Ancestry race, which can only be found in the current lockboxes or at the AH north of 15M AD, unlocks on live as well when unlocking it on the preview server. Reports vary: Some say you also need to have the Dragonborn race available and get a character through the tutorial while other insist it’s available as soon as you open the Sigil of the Metallic Ancestry Dragonborn.
People have actually started selling the access to the Sigil on the preview server for ADs, so be aware!
** UPDATE ** As you might have already figured, this was our contribution to April Fools’ Day. You can’t unlock the race this way, sorry to all those that actually tried (probably no one). But this also has a more serious note attached as we’ve received trustworthy word that the rumors swirling around since January are indeed substantial and an unlock on the preview server carried over to the live server for a certain time after release. They allegedly fixed this during an emergency maintenance soon after the linked posting.