Don't think this has even been referenced yet, so might as well quote it in case someone's missed it. From the very first DFtalk back in 09:
"Rainseeker: Actually if you want to do some older ones I can grab some more old ones I've asked. Here's one [Kilo24]; 'can souls be applied to inanimate objects and/or as part of artifacts?'
Toady: So, for the current release - the one that's coming up - there aren't going to be souls beyond what I just added for the creatures. Right now the creatures can hold a number of souls but in practice they only hold one; that was to support possessions and so on in the future. But really all the soul is in the game is, it's got a name, it's got some historical information, the main thing it's got is skills and the personality, and the idea behind that kind of separation was to allow them to be put almost anywhere. They can be floating around in some kind of ghost or they could be stuck in an item, some of the framework's not there for that but they were created with that in mind, so that if you wanted to have some kind of artifact singing sword that hollered things as you walked down the dungeon corridors, and then you could stop and talk to it and ask it if you want to go left or right, and it'd be like 'I'm scared and I want to leave' or something, you could do that, and it wouldn't be that hard to set up, actually. It's in the future, certainly way in the future perhaps, but the framework is building up for that."