As in, during the next expansion. When a player asks what happened to Jaina and the Missing Diplomat, Drysc mysteriously says "Northrend holds many secrets, and many answers." So it's a good guess that we'll find out at least another piece of the puzzle to where the Lost King of Stormwind has gone.
And it's also a pretty good guess that Jaina will be there, somehow. Despite the fact that she's pretty much the strongest leader the Alliance has, she's also pretty buddy-buddy with Thrall, and she's got that history with Arthas-- they were friends (and possibly romantically linked) before he went all power-crazy and possessed. There's no question that some of the answers we uncover in Northrend, whatever they may be, will involve Jaina Proudmoore.