It also sends you email confirmation that you got the sigil. They had this running prior to this, having one hero per week or few days. Looks like they opened it up to all.
Thank you for participating in the Diablo III: Darkness Falls, Heroes Rise Mark of Valor campaign. To reward you for your participation, an exclusive in-game sigil representing your hero has been awarded to the Battle.net
account registered with your email address. Shortly after Diablo III is officially released, you'll automatically have access to this sigil whenever customizing a character banner.
Don't forget, each week a new hero will be featured, offering you the opportunity to unlock additional sigils. These items will only be available for a very limited time, so be sure to heed the calling of all Sanctuary's heroes!
Prepare yourself for battle,
The Diablo III Team
To learn more about this promotion, see our official FAQ.