It's possible to implement in vbulletin (in fact, there's already mods written to do just that). It wasn't possible to implement in a sane way in Fusetalk, which was why they went away.

I'm not philosophically opposed to spoiler tags. I see a lot of other potentially good uses for them -- like hiding giant layout-breaking images in the desktop thread, for example.

However, I am opposed to the idea of spoiler tag abuse -- if we were to install such a mod, I'd expect that discussion rules would mostly stay the same: manga spoilers still wouldn't belong in anime discussion threads, and vice-versa. I'd also expect that there wouldn't be large stretches of threads wrapped in spoiler tags. However, I'd personally be a more active poster in a couple of threads (series discussions for series that I'm not quite current on) if I knew that clicking on the discussion thread didn't necessarily assume that I'd already seen the latest speedsubber-release of the series.

Take, for example: utawarerumono. I'm watching yesy's version, and don't really approve of your-mom's approach to subbing in general (they're sort of ... spitesubbers), so I don't intend to watch theirs. Since I'm waiting for a relatively reputable group to release, and there's discussion of your-mom's episodes in that thread, I can't really participate meaningfully in the thread anymore. By the time yesy twitches again and releases another couple eps (or, I guess if Static does underground releases), most discussion will have already happened about your-mom's version, meaning it'll still be pointless to even try to participate in the thread anymore.

Spoiler tags make enforcement of spoiler rules a bit muddier, and make discussion moderation in general a bit tougher. But with some sane rules, they can help prevent that from happening by letting people watching the speedsubber versions of things stay in the discussion while not spoiling and giving a generally bad time to the non-speedsub-watchers, and I'm all for that.

So, really, I'm on the fence. I'm interested in seeing how much interest there is in this ... it's really not particularly hard to make happen, just a question of whether we want it or not.