Crygil says that, actually, they just might:
"Who said that the Alliance wouldn't attempt to reclaim these lands? That is a very dangerous assumption to make, especially if you're of the forsaken persuasion.
The Lich King is a threat to all the inhabitants of Azeroth and, as such, he needs to be put down before territorial disputes can be resolved. Leaving those claims for the future only makes sense; far easier to fight one war at a time. For now, it's better to focus on the task at hand."
Makes sense. So far in Cataclysm reveals we've mostly seen the Horde making new conquests, moving in on Alliance territories, and constructing new strongholds, leaving us to wonder what exactly the Alliance is doing that's taking time away from actually defending their footholds. But we've also barely seen any information about what's happening in zones outside of Kalimdor. With the opening of the Greymane Wall and reemergence of the nation of Gilneas, suddenly the Alliance has a foothold in the Lordaeron subcontinent again -- one that the forsaken seem desperate to reappropriate.
When you take into consideration the Cataclysm zone overhaul map, which shows the Plagueland zones being the recipients of considerable makeovers, it seems that we're likely to see a reclamation of the blighted lands. If they were Alliance territories again, they'd be a fantastic springboard for an Alliance invasion of Tirisfal Glades, with the forsaken stuck between a rock (Gilneas) and a hard place (Lordaeron).
Full disclosure: I play Alliance almost exclusively, and I think this is great. While I don't expect to see an all-out assault on the Undercity, at least at this point in the game's life, it'd be a damn fitting comeuppance to have the openly aggressive and devious forsaken be on the defensive for once. From Crygil's response, we might not be that far off.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.