Click to enlarge photo. That dome looks to me to be protecting the Navy's LaWS technology demonstrator, a 33 kW laser that has been successfully tested many times and is on pace for IOC in 2017 if the Navy chooses to go with the program. This laser would be a replacement for CIWS or RAM. The per unit cost is estimated at $15 million, and it doesn't have the logistics tail for ammunition that CIWS and RAM have.
I see generators on the flight deck, and what looks like a laser designator mounted on the back of the flight deck. I am very curious to know if there is additional equipment in the hanger bay for cooling, or if it is all contained in the equipment of the dome. Keep in mind this is one of the smaller lasers the Navy is looking at for fielding to the fleet, which is why the CNO is being very smart about the DDG-51 Flight III development and making sure those who are pushing that design are thinking clearly about what the next major combatant really needs in terms of space, power, and cooling for future weapon systems.
This whole system looks very big for back-fitting on DDG-51s, but there is a US Navy vessel out there that can take on all this kit (and more) and field a larger laser - both versions of the Littoral Combat Ship. The LCS may never field ESSM as some advocate it should, but I won't be surprised if LCS fields laser air defenses by 2025, although I hope it is something bigger than this.
If you are looking to learn more about the US Navy's developments with lasers on ships, I highly recommend Ronald O'Rourke's Congressional Research Service report Navy Shipboard Lasers for Surface, Air, and Missile Defense: Background and Issues for Congress (PDF).
I have been advised the DoD will be discussing this in more detail on Tuesday, hopefully including more photography. While the DoD may say this has some association with RIMPAC 12 or some other ongoing activity, I'm thinking they would be smarter to note this is timed with the 25th Anniversary of Star Trek: The Next Generation celebration, because as you know - that's a big room full of nerds outside the Pentagon that would probably appreciate this development.