With the old world changing so drastically come The Shattering, there's been some question of how the Loremaster achievement might change in the wake of Cataclysm. Community Manager Zarhym has clarified this for us in a recent blog post. As he puts it, if you already have either Loremaster of Eastern Kingdoms or Loremaster of Kalimdor, those achievements will stay intact. However, if you're only partially through with those achievements, you may suffer a setback, as any quest that's being removed from the game will no longer count toward those achievements in progress. Therefore, it's a good idea to finish up the achievement if you can.
That said, it will be a little bit easier to get those Loremaster achievements come The Shattering. Instead of having one huge quest counter for each continent, patch 4.0.3a will introduce zone-specific quest achievements to the old world, similar to the ones that already exist for Outland and Northrend zones. Once you finish the zone-specific achievements for every zone on a given continent (minus the new Cataclysm zones, of course), you'll have the achievement. Even old-school Loremasters can get in on the action, as the zone quest achievements will still be available for them to complete.
While Zarhym didn't specifically outline it in his post, Loremaster of Cataclysm will be required for future Loremasters as well, although it's not clear whether that will start to be a requirement in patch 4.0.3a or at the launch of Cataclysm itself.