... and will continue to do so in the Expansion.
The question of which spec to go for when levelling a new PvE Alt often comes up in blogs and forums, and most often the readership comes back with Prot. Now, there are a lot of reasons why Prot is a great spec for levelling but I just think Ret is better. Here's why:
Experience - Quests in World of Warcraft contribute a whole lot more experience to your level since 2.3.0. Previously a certain amount of grinding was expected to stay ahead of the levelling curve, but that's no longer necessary as quests provide more XP than before and the XP/level has been reduced. As questing most often requires killing specific mobs, often before respawns, I've found that Ret is most efficient. Casters - Retribution is much better at dealing with casters than Prot or Holy, and I've always had more difficulty with them than physical damage NPC's. Fewer Elite (outdoor) Quests than before - Holy and Prot had a big leg-up over Ret when dealing with Elites, with fewer Elite quests this advantage is eroded. Durability - In my experience the durability of a Paladin doesn't significantly drop when going from Prot to Ret when questing (group play is obviously another matter). Personal Satisfaction - Ret of course throws up the big yellow numbers... I like big yellow numbers... you do the math ;l). The spec generates it's own momentum - Every talent point you invest on the way to 60 is useful and provides a tangible benefit. It has a high quality 41pt talent giving you a clear target to aim at talent-wise. Vengeance, being a limited duration buff, provides me with an impetus to kill and keep on killing. Pursuit of Justice allows you to run faster 20-40 and again increases the rate at which you kill. The same cannot be said IMO of levelling as Prot nor Holy. Verigan's Fist - If you are lucky enough to be Alliance (and why wouldn't you be?) Verigan's Fist plain rocks. It is peerless lvl20-30, and only Ret can exploit it to its fullest.
Retribution is not without it's downsides. You really need to keep ahead of the gear curve, preferably by having a store on Rare BoE 2-handed Weapons or having friendly guildies take on an instance tour at key levels. Taking up Mining and Blacksmithing/Engineering can be handy for non-weapon gear, but generally you'll gain more from quest items or the AH especially if you can do a few quick instance runs. The flip side is of course that Ret isn't as desirable for low-level instances, people generally prefer Holy/Prot, but if you don't expect to be doing instances with people around your level this need not be a consideration.
Once you hit ~lvl62 you should consider swapping to the spec you intend to do at end-game and start learning your role in instances. It's not exactly raiding but it does serve to educate you in how your spec works to a much great extent than levelling ever could.
Putting my money where my mouth is, I'll also be levelling as Ret in Wrath on at least one of my Paladins, possibly both if I decide not to tank any 70-78 instances.