Is that a popcorn ceiling? Popcorn will drop right off if it gets moist sometimes. Painting it will cause it to come down in sheets like you described if it's not adhering good to begin with. The best thing you can do is try to lock it in place with a good oil based primer then topcoat with an acrylic latex. BIN is a shellack, and I tend to think of oils as a better glue for stuff like this.
The problem is the popcorn itself...Assuming this is what's on there. Usually other textures hold up real good. (My luck, you're talking about a smooth ceiling, hah) Popcorn texture is made with a chalk base...NOT a mud. The chalk dries out and doesn't adhere extremely well. Hit it good with moisture once, and it's a goner! Painting them is hit and miss always, and you need a clause in your contract for them, if you ask me. (been burned by em allready)
It's always a good idea to test for adhesion by dampening a sponge and soaking a few suspect spots on the ceiling. Also, giving a little scrape here or there with a fingernail will give you an idea how good it's holding. I just don't guarantee that ceilings like that will hold once painted. The adhesion of the texture to the substrate also falls under "pre-exhisting conditions"!
