Always. Lycanthropy is the disease you only get once.

Apparently since Skyrim. New disease, not having to go to a witch coven to permanently cure it.
^ That's the cure method, for anyone who hasn't found it already.

In past games, curing vampirism would prevent you from catching it again.

It isn't exactly sunlight. It's a glimpse of Aetherius. The day is bright because the hole Magnus tore in reality allows magic to filter down from Aetherius; you can 'see' it as light, but there is also magicka coming through the holes. The night is dark because you are looking at Oblivion, with only the smaller holes torn by the Magna Re piercing through to beyond. Convoluted lore reason for getting hurt underground in the day sort of provided, almost, a bit.

Well I knew about the process to cure it, I just wasn't aware that you could catch it again. Thus far, I'd assumed it was like Oblivion, & Morrowind. And you know, curing it rid you of it forever. Could you get it cured in the same manner more than once? Because that would be a bit ridiculous.

Yes, you can. And with Dawnguard, you'll be able to catch lycanthropy more than once as well, for those who gave it up to try out the vampire lord.

Source?

E3 Interview, I think. I don't have the link, but I know that one of the directors said it. Plus, with the lycanthropy skill tree being added it wouldn't make sense (gameplay-wise, not going into lore here) to not let an established character reexperience lycanthropy after trying the vampire lord.

Huh, that must make for some interesting moments. Hand him a black soul gem, "It happened again."
Also, I wonder where you're supposed to reacquire lycanthropy? I guess that might be something of an unofficial confirmation of wild werewolves? Or maybe you just ask a certain lady archer to open a vein?

The Gametrailers interview I'm pretty sure, don't hold me to it.

Well, there are already non-guild werewolves in the game (in the silver hand bases and Ill Met at Moonlight quest), there just is no Saines Lupis(?) disease mechanic attached. I'd love wild werewolves with a catchable disease, but I think the latter of what you said is what they're going for.

My guess: the blood won't go away, allowing you to drink it any time.

That's not quite as fun as having to pester the members of the Circle for a sip any time you want a bit of wolf blood.

Probably. As of right now, when you cure it, Aela shuts you out of the Underforge. It seems really cop-outy to do that, but whatever; it's Bethesda's game, they can do what they want.