Some thoughts that support the used 747 rumor.

1. An established pattern of Delta buying used airplanes that are no longer in vogue pennies on the dollar. (SWA owning airplanes the past few decades is one thing that gave them a leg up on the competition). Low or no monthly payments is a good thing in the airline business.
2. Delta management has touted the benefits of having low cost iron on the property. This allows:

a. Flexibility to quickly adjust to market conditions and route opportunities. You can park the planes with no real penalty (other than paying the aliens for ramp space in Roswell) when the economy declines and dust them off to ramp up when needed.
b. Having low acquisition cost airplanes allows for more spares ready in wait to clean things up after the shoot hits the fan. The 747 is large enough to do a lot of clean up after storms where the new MO is cancel a ton of flights in advance then shift into high gear to get people on their way. The 747 can step in to a given route where say 5 flights to that destination have been cancelled. Get all the backed up pax to that destination on one airplane is a good thing.
c. Extra 747's allows the ability to pick up more charter work (which the company has been upbeat on lately) on short notice without disrupting other routes.
d. The comparatively low operating cost if you fill em up.
e. Should the company decide to reverse the "there is no money in cargo even though Fedex/UPS are minting money" the 747 purchased at a low cost can fill this need. Asia (specifically China) could produce some great opportunities in freight as Delta looks to expand into this part of the world.
f. When Delta decided to spend the money on the lie flat seats and upgrade the current 747 fleet they committed to at least another decade of 747 use at Delta Air Lines. Adding more numbers to any existing fleet typically brings cost down.

777-300's will do a nice job and these I am sure will come to Delta as well but for pennies on the dollar the 747-400 could fill a lot of gaps for a relatively low acquisition cost.

Food for thought whether it happens or not.