I don't have any Delta travel coming up, but I recently read that Delta is now selling electronic giftcards. On a whim, I decided to check to see if purchasing a Delta e-gift card would trigger the $50 statement credit. I purchased the e-cert on Friday and today the credit posted! Now I just have to remember to use the e-cert to pay the taxes and fees on my next Delta award flight.
UPDATE: Thanks to DeltaGoldFlyer's comment, I see that Rene over at DeltaPoints has confirmed that $50 Delta E-gift cards qualify for the Plat. credit!