Lisbon Oceanarium, Lisbon (Portugal)
In 2002, after two unsuccessful attempts, the aquarium received a manta ray (Manta birostris), 2.2 m wing-span (DW), caught in a set-net for tunas in the southern region of the country (Algarve). At that time, no other aquarium outside Japan had ever been successful in displaying the species. The animal currently swims in the 1.22 million-gallon "Global Ocean" tank (34x34x6.7m; water temperature 22º C), together with 80 species from around the globe. It feeds on small crustaceans once a day by filtering from the water the food given by staff members. Two years later, a 1.35 m DW male devil ray (Mobula mobular) was caught in the same set net with wounds in both cephalic lobes and tail. It has recovered totally and is still the only one of this species found in any aquarium, measuring now more than 2 meters DW. The large manta ray will be released this year, between spring and the end of summer.
Update:
"The male specimen was released on April 12, 2007, measuring 3.5 meters wing-span, after spending more than four years at the aquarium facilities. The operation involved huge logistics, never used before by any aquarium, such as a special crane adapted to the building's structure and a floating crane holding a large container where the animal was taken to the release site, 17 miles from the aquarium (never an animal was released this far from its captive facility). Two sattelite tags were attached to the animal's body, and for six months the aquarium staff will be receiving informations and data about its location, the depths it has plunged, etc. The other specimen, a devil ray (Mobula mobular), is now the major attraction of the Open Ocean tank."

Okinawa Churaumi Aquarium, Okinawa (Japan)
The new facility in Okinawa, the suceesor of the Ocean Expo Aquarium, continued the display of manta rays in their 7.5 million-litter "Kuroshio Sea" tank. They exhibited successfully four specimens of Manta birostris with wing-spans from 3.5 to 4.2 meters and innitiated a breeding programme for the species. On June 8, 2006 the specimens in the tank displayed mating behavior, for the first time anywhere else in the World, and the aquarium's staff hopes to have a newborn this year.
Updated by Filipe Pereira:
After recording mating behavior last year, a manta ray pup was born on 16 June, 2007; the first time ever in captivity. See below for details and pictures.

Georgia Aquarium, in Atlanta
has announced on 25 Aug 2008 their new resident in the 6.3 million-gallon "Ocean Voyager" exhibit, ) brought from "uShaka Sea World", in South Africa, in a 30-hour long operation. About a year ago, the female specimen ("Nandi") was caught in a shark-net in Durban nearly dead and was transported to the park facilities to be treated for wounds sustained during its struggle to free itself from the net. At that time, she measured almost 8 feet (2.4 m) wing-span and was kept in a 580,000 gallon open air tank representing the oceanic habitat. After one year, the animal outgrew the exhibit and the staff wanted to release her back in the wild. Two biologists that cared for her decided to call the Georgia Aquarium if they were interested: A team of the Georgia Aquarium flew to South Africa to prepare the whole operation of transfering "Nandi" from Durban to Atlanta. In July 2008 , "Nandi" arrived the GA Aquarium, where it was kept in a holding tank attached to the "Ocen Voyager" exhibit, ideal for training her to feed properly. August 25th was the day when "Nandi" was finally introduced to her new home, shared by four whale sharks and thousands of other marine fish, where she will be observed by millions of people from this day forward, as an ambassador of one of the most fascinating species of our oceans...!
Update by Richard S. Nugent: Georgia Aquarium now (March 2011) has four mantas on display. The original (Nandi) which came from South Africa and three others (two female, one male) taken off the east coast of Florida between 2008-2010. They range in size from 10 to almost 12 feet across. All are doing well and are still growing.