http://dmn.delta.com/offers/janbday/

Fine print states, First register, then book your round-trip ticket by January 31, 2011 and complete your travel by February 28, 2011 to receive your 2,500 birthday bonus miles.

My trip was booked PRIOR to receiving the email and registering. I still received 2500 bonus miles for an existing itinerary after I registered. Thanks Delta!