If you're reading this, that probably means you're already somewhat ahead of the pack by using resources found outside the game (or you just like reading sites devoted to World of Warcraft for some odd reason) instead of ouido-ing your way through Azeroth and not unlocking your full potential. In a way, it's sad, because players who simply want to enjoy playing a warlock or some other DPS class will eventually have to get those proper rotations or sharpen up on game mechanics in order to be respected or invited to groups (the dungeon finder notwithstanding). Warlocks, in particular, have a steep learning curve, having to watch debuffs and avoid clipping DoTs (you can still effectively clip them, even in 4.0.1), among other things.
Fortunately, there are a lot of resources out there to help you should you feel inclined to do the work. Forums such as Elitist Jerks have a community of number crunchers, and there are a lot of players who have excellent warlock blogs that help round out your game. I wish I could say you don't have to go outside the game to play better, but the awful truth is that in order to eke out the most from your warlock, you'll probably need to draw on the collective knowledge of players everywhere. This is largely why I prefer to PvP. Because I hate doing the math. So today, there won't be any math. I hate looking at graphs and jotting down my numbers on a target dummy ... but I know I have to do it. You, if you're a "responsible" warlock, will need to do it, too. I'll try to help out by telling you what you'll need to understand in preparation for Cataclysm.
On the DoT
Warlocks have their work cut out for them because damage over time isn't a simple thing. Avoiding clipping DoTs has always been one of the more challenging aspects of the class, and not everyone masters it, especially without addons. It used to be that you would err on the side of caution and just let the DoT fall off before casting again, just to avoid overwriting that last tick (or "clipping" it). It's a bit easier post-4.0.1 because instead of overwriting the existing debuff, the latest cast spell is tacked on to the old one and will just start ticking after the existing spell has ticked. Sounds great, right? Now you don't have to worry about clipping DoTs!
Well, not necessarily. You can't just, say, cast two Corruptions in a row thinking that the second Corruption will start ticking after the first one finishes. That's not the way it works. Instead of clipping DoTs and starting to tick, the newly cast spell will extend a courtesy to the old spell and let it tick once before it starts its own duration. It won't clip the older DoT, but it won't let it finish its full duration if it has several more ticks to go, either. That means you should be reapplying the DoT right before the last tick of the existing DoT. That actually seems harder than the old method, where you could just watch the debuff disappear from your target before casting again. The good news is, in practice, you won't actually need to do this with a whole lot of spells because they refresh automatically, thanks to another effect such as Haunt (through Everlasting Affliction). It's good to know, anyway, because there will be situations in which you'll probably need to reapply DoTs manually (such as some spells such as Haunt has a travel time, so there will be rare occasions where Haunt will land too late to refresh Corruption).
Haste and its evils
Typically, warlocks like to embrace evil stuff. In this case, though, I wouldn't recommend it because haste is evil. Evil, like the fruits of the devil. Don't get me wrong -- haste is great for DPS. You want haste. In fact, if you're demonology or even destruction, you'll want loads of it to the point of gemming and reforging for it above all other stats once you're hit-capped. But haste doesn't help in a linear, straightforward fashion, which I honestly think is pretty evil. (OK, fine, maybe not evil, but it's certainly annoying.) Haste has been adjusted in 4.0.1 in a way that doesn't shorten the duration of debuffs the way it did in Wrath. Instead, DoTs will retain the same duration but haste will try to squeeze in extra ticks depending on your haste value.
Haste is pretty powerful stat in Cataclysm, and you'll want loads of it. It reduces global cooldown and spell cast times, and grants the aforementioned extra ticks on your DoTs. The problem is that this new method of trying to squeeze in extra ticks into a (more or less) set duration means you'll need to reach certain haste thresholds to get each extra tick. It won't matter if you're just 1 haste away from the next threshold; you're not getting that extra tick. See how evil that is? Those folks at Elitist Jerks who do the math say the haste thresholds to hit are 157; 781; 1,406; and 2,030. You'll have to test out those values yourself on a dummy, and even then, numbers can change depending on your buffs. Your mileage may vary.
The bottom line is that haste has changed, too, just like DoT refreshes, so you'll want to brush up on that -- at least, until haste values change for level 85. One other thing to note is that spell durations change a little bit, depending on how many ticks your haste can squeeze in. If you hit that threshold and haste inserts that extra tick, your DoT duration can actually go a little long. I told you haste is evil.