I haven't seen any news about what happened to Diego Garcia, bearing in mind this is a highly secret US base on British territory. According to the US military (see link in title) US Navy personnel on board Naval Support Facility Diego Garcia in the Indian Ocean are safe following the Dec 26th earthquake and subsequent tsunami that had devastating effects on Southeast Asia. Facilities and operations were not affected, at least according to official sources.
Favorable ocean topography apprarently minimized the tsunami's impact on the atoll. Diego Garcia is part of the Chagos Archipelago, situated on the southernmost part of the Chagos-Laccadive Ridge. To the east lies the Chagos Trench, a 400 mile long, underwater canyon that ranges in depth from less than 1,00 meters below the surface to depths that plunge to over 5,000 meters. It is one of the deepest regions of the Indian Ocean. Diego Garcia is located to the west of Chagos Trench, which runs north and south. The depth of the Chagos Trench and grade to the shores does not allow for tsunamis to build before passing the atoll. The result of the earthquake was seen as a tidal surge estimated at six feet.