Scrape the popcorn with a putty knife. It won't take the popcorn off but it will chip off some of the paint. With some paint off a spray of water will get to the popcorn and soften it for removal.

jan9 is exactly correct...I have removed many popcorn ceilings that have been painted and those that have not. YOU MUST SCRAPE THE TIPS OFF BEFORE YOU SPRAY. I wouldn't scrape if it hasn't been painted though, because in my opinion those chunks of cardboard like bumps help absorb water. If you scrape like he says, then spray the bajeebies out of it (use plastic under a big drop cloth) and let it soak in, then spray again until it is totally saturated, it will actually come off in "sheets". I found that I had to soak/spray for a good 45 minutes or so with painted over ceilings. When the water starts dripping, let it soak in and keep spraying, let it soak in,...etc.
Don't try to get it off until it "gives up" or you will make a gigantic mess for yourself. Keep a big trash can with a bag next to you and drop the sheets into it. After a good dry, I skimmed my ceilings with drywall and sanded with drywall screen and the pole, primed with Zin.123 and painted. It's hard work,...I kid you not. Worth it though to get cottage cheese off the cieling! It makes the ceilings look one foot higher. I once did an entire house.