In Halo 3, one cinematic states that Cortana has found a solution to the Flood other than the Halo rings. Master Chief wants to go to the Ark to pursue this solution (and is told that stopping the rings is first priority).
According to this question, the Gravemind also wants to know what this solution is.
However, by the end of the game, as far as I can tell it hasn't been mentioned again (tvtropes warning), and the game ends with the Flood still presumably at large.
Did I miss something? What was this solution? Does it get mentioned again in a later game? (For bonus points, what did happen to the Flood elsewhere; are they just being "contained" using "conventional" means?)