Tseric answers on the forums, and says that while Blizz is leaving their option open, the main problem with returning to unfinished quests (The Missing Diplomat/King of Stormwind is another) is that they'd have players of a high level doing low level quests. Either that, or they'd have to interrupt the normal leveling game, and you'd be doing quests in the same line with ten (or more) levels between them.
But while that's an obstacle Blizzard is considering, apparently it's not going to stop them from revisiting some old questlines: reader anobolic (thanks!) sends a link to this flickr account, with what looks like an extension of the Quel'serrar questline!
If you've got Quel'serrar now (not sure yet if Blizzard will give it back to you if you've sold or destroyed it for some reason), you'll be able to do another set of quests (rumored to involve another dragon, and maybe even an Undead Onyxia-- she's back?!?) that will either get you the Daughter of Quel'serrar (more purple for paladins), the Son of Quel'serrar (for warriors), or, if you're the first person on the server to complete the quest, Quel'serrar Reborn (and apparently you get a "dragonslayer" title with that as well). Oh, and there's also a rumor of a Quel'serrar shield to come. Very cool! Of course, all this is rumor so far-- it's not even certain that this is
Update: Commenters are casting doubt on the authenticity of the pictures. Stuff like this could be easily photoshopped, and we have no other source for it, so take the info about Quel'serrar with a grain of salt.
Update2: Commenter Leggend (thanks!) says this is proof the images were just adjusted in Photoshop. Oh well. Maybe someday Blizzard will actually finish off some of these old questlines.