If you missed out on getting any of the Diablo 3 exclusive sigil, now is your chance! With all the classes now unlocked in the "Darkness Falls, Heroes Rise" game, you should move fast and get the sigil for all the classes.
You should hurry, as the opportunity to get the Diablo 3 exclusive sigil will only last until one week after release. The process is very simple, simply choose one of the classes shown above and hit next to go to the sigil generation screen.
Once on the creation screen, you have various options that allow you to customize your banner. You can upload a custom photo, change colors, pattern, etc, or just hit randomize to have the creator do the work for you. The sample above is from the Witch Doctor sigil creator. Once you are happy with your design, hit next to go the next step.
It is very important that you enter the correct email associated with your Battle.Net account on this step as the email isn't actually validated to check if it is belongs to an account. Once this is done, you will receive a confirmation email saying you have unlocked the Diablo 3 exclusive sigil for the class.
According to this forum post, all sigil requests will be processed after the initial release so you can expect you sigils in the first 2-4 weeks after release.
We're keeping records of all the Battle.net accounts which have claimed each sigil. Within the first couple of weeks of Diablo III's release on May 15, we'll be processing all of these accounts and flagging them to unlock the appropriate sigils. The bonus sigils will automatically appear in-game in the banner customization screen in-game once awarded.
Update (29th October): Since that time I have had all of the exclusive sigils unlocked on my account so I figured it was about time I updated this post and show exactly what each one looks like. If you missed the opportunity to get these exclusive sigils, here is what you lost: