I seem to recall that abandoned worldgen fortresses are supposed to be reclaimable, but
only fortresses since they are guaranteed to be small enough to be playable (all the other sites can go up to 17x17 or something), with a possibility that abandoned human forts might be reclaimable in the near future as well (possibly even for this release, but no solid word on that yet).
Worldgen dwarf fortresses are almost certainly reclaimable:
Threetoe: I was thinking, you'll be able to reclaim your own fortresses and now the actual mountainhomes themselves that the other dwarves created.
Toady: Yeah, there's going to be just a menu of interesting options right from the beginning of the world, if the dwarves had a hard time of it and their whole civilization was wiped out.
Other dwarven sites are definitely not reclaimable:
Toady One: I'm working on deep dwarven sites, and it should include a bit of early work on fortresses as well, since the basics of their individual living arrangments and industry should be about the same. Deep sites claim areas much larger than forts (the size of human towns in the current version -- up to 17x17 embark tiles), and a lot of this will probably be devoted to farms in the cavern layers, but I'm hoping for something reasonably different from a human village underground. These kind of sites aren't reclaimable (because they are too large), so the amount of farms won't clash with the time-compressed nature of farming in fort mode. Aside from the living, farming and workshop areas, there'll be zones for mining and storage as well. Certain of them have barons, and those'll have an additional area.
Musings on the idea of reclaiming ruined human castles: (not the quote I was looking for but I can't seem to find it anywhere)
Rainseeker: (Indiana Jones theme)
Toady: That's right, exactly. The proper ruins from older civilizations, and we're going to have to go through world generation and make sure that there are enough wars and famines and plagues and migrations and so on to make sure that there are nice isolated hidden ruins, then you'd be able to go into those. In dwarf mode, right now all you can do is reclaim your previous fortresses, so there are a couple of angles here. Once there are actual fortresses with maps, which would be another adventure mode thing, just being able to go visit a fortress, then opening up reclaim on those is a lot more legitimate. Right now we just can't do it because they don't even have maps, you'd be reclaiming a blank cliff face and there'd be nothing there. So once that's handled then we can start doing that. I like the idea of a historical reclaim, it's just kind of a start scenario; you shouldn't just be able to reclaim your old fortresses you should be able to go to old ones. The only downside is that maybe you'd be getting massive amounts of treasure you don't deserve, but then there should be proper guardian beasty type things and trouble.
Rainseeker: Or else it'll already be scavenged.
Toady: There's kind of a risk/reward thing going on there that should naturally come out of how adventure mode works, and if the place is so isolated and hidden that there's treasure there with no-one guarding it then you probably wouldn't be able to reclaim it because you don't know about it, until someone finds a map, or whatever. So that's the situation there. I think when it comes to other weird ruins, like the ruins of a human civilization, I don't know if there's ever going to be like a dwarf mode colonization or claiming of that. It's similar to how you can't just settle inside a human town with a dwarf fortress anymore. But I'm not sure, it's certainly not as off limits as that, it would be a kind of legitimate thing. You'll probably more likely find an adventurer making a bandit camp there or something, but if they can do that there's probably no reason why the dwarves can't go to an old human castle and set up a mine underneath it or whatever. That'd be kind of fun.
Rainseeker: Yeah, especially if you went inside the castle before you started digging.
Toady: Yeah, and you could clean it up. It'd be a fixer upper. And you could rebuild the walls, and kill the zombies and stuff.