Do you or a friend have a September birthday? Try your luck:
2,500 mile version of the monthly birthday bonus is now live for September birthdays: http://dmn.delta.com/ace_offers/bdaysept/
2,000 mile version: http://dmn.delta.com/ace_offers/septbday/
1,500 mile version: http://dmn.delta.com/ace_offers/sept_bday/
If you haven't checked your Delta profile recently, go to Profile, then scroll to the bottom of the default About Me tab. They've cleared out some information to refresh TSA-related stuff (at least on some profiles I manage), so be sure your birthday is saved correctly. Could be used for targeting birthday bonuses.
No matter what, if you don't get an email saying you are qualified for the promo, try to register during the valid period (usually begins a week before the promo month) for your applicable birthday month anyway. Start with the highest mileage offer, then work your way down. Can't hurt. Don't, however, expect that you'll necessarily qualify. Delta may very well have criteria beyond simply having a birthday, and it's their prerogative to do so. Remember - it is a bonus, not an entitlement. Seek birthday bonuses from another airline if you aren't happy with Delta's!
* NOTE TO FLYZABIT
: The summary page currently lists just two link formats for birthdays. The past few months have seen the three above. Might I suggest updating to include all of them (2,500, 2,000, and 1,500 mile offers)?