I haven't lived with plaster walls before, or if I have, it was in a place where the landlord said, "Don't nail anything to the walls." Who knew there's a reason you're not supposed to nail into plaster and lath?
I experimented with hanging a picture in our front stairwell today, and as I pounded in the nail, I heard a shower of plaster inside the wall. That can't be good. Did I stop? No. I gave it one more bang to complete the job and then Googled "plaster and lath walls."
First thing that pops up was Old House Restoration, where it says "NOTE: When you want to hang a picture, never use nails in furring strips! Pounding a nail into the wooden furring lathe can loosen the keys of the plaster and cause sagging or bowing of the wall. For that reason, always use screws in plaster walls, predrilling each hole. A piece of cellophane tape over the spot you want to drill can keep the surface of the plaster from spalling."
Now I'm all paranoid that I've done some major harm, that the wall will fall on me next time I walk past it, and we've only had the place since December. Thankfully, the link above gives some information on how to correct it with plaster washers if the plaster actually is pulling away now.
Additional link I should have looked at before whipping out my trusty hammer:
Repairing Historic Flat Plaster Walls and Ceilings