Yes – surprise, surprise – Ada Wong is totally a playable character in Resident Evil 6. Capcom (finally) confirmed as much to Famitsu, and released the video you see above of Ms. Wong in action. It seems her portion is only playable once all three other campaigns are finished.
In addition to Wong's big reveal, a new mode dubbed "Agent Hunt" was announced, wherein players control either one of the game's four protagonists or one of two enemy characters (a B.O.W. or a J'avo, according to Siliconera). We expect we'll see more Western-geared variants of these trailers in the not too distant future. For now, enjoy the Japanese versions.