Jim's comment is accurate and I was wondering about the DOJ too. For US Airways and Delta to close the transaction before the DOJ made an additional public announcement should mean the two companies have confidence the DOJ will not object.
The DOJ has no authority to block the deal and according to Scott Kirby in a comment made during the 3rd quarter Earnings Conference Call and earlier in the week during the CLT Pilot Crew News session the only way for the DOJ to stop the deal is for the regulators to file suit in federal court.
In my opinion, this is the first of a couple of more steps in the DCA-LGA (and BOS) markets. Additional strategic initiatives could be the re-purchase of the 10 E-190s/DCA slots previously sold to Republic, the E-190s becoming the only Shuttle aircraft in all 3 markets, code sharing the Shuttle with American, and if the FAA Reauthorization bill provides additional beyond perimeter exemptions US Airways could obtain additional slots and/or convert existing slots to fly from DCA to SAN, LAX, & SFO.