Simplest (best!) thing to do is get the ceiling replastered. Got a 900sq ft apartment ceilings done for around €1000, and seeing the results now versus before, it was money very well spent.
Tried several ways to do this before giving up and getting the plasterers in.
1. You're told that you can wet the stipple and scrape off. Works okay, but took half a day to do about 20sq ft. And working over your head all the time isn't very nice. Problem as mentioned above is scraping down too far. Also, it's very difficult to get a smooth surface.
2. Tried a sander as well. Started off with a small hand one, but the dust was unbelievable. And again, working over your head is a pita.
3. From ringing around, there is only 1 sander (that's 1 actual sander, not 1 model of sander) that is suitable for doing this available from tool hire places. It's like a big hoover that you can hold up and push against the ceiling. Is available from SAM Hire (I think), but they messed me around so much, I gave up looking for it in the end.
The plasterers did the whole place in a day. Wet the ceilings first. Used an "industrial sized" scraper - large rigid metal plate on end of brush handle - to take the bigger bits of stipple off.
Then replastered. Fantastic job in the end.