Update: All Class Sigils have been awarded. If you don't have them by this point (and the associated Feat of Strength), you're out of luck!
You are credited for them already.
The thing is, Blizzard's servers were swamped (surprise!) when they first started the Mark of Valor, and since you did not have to log in to Battle.net to partake in the activity, the only record they had of you taking part was if you correctly entered your email address in the final step.
Now, even beyond that, there were some issues with verification - if you did not get an email confirmation of your Mark of Valor submission, Blizzard has lost your entry into the aether, and has no record of your work.
But not all is lost! You can go back to the Mark of Valor page and reacquire any sigils you're missing until the page closes on 5/18/12.
We now have a blue post with a less nebulous date:
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.