Don't worry about tiling the ceiling. It's not a big deal. Use a modified thinset (Versabond from Home Depot is fine). Modified thinsets are noticeably stickier than regular thinsets.
The key is the surface. Fresh drywall (not greenboard) works real well for a ceiling. IT's best not to prime it or paint it. Just leave it raw. Especially, don't use PVA drywall primer. Thinset doesn't stick well to that stuff. If it's already painted, hit it with an orbital sander to scar it up, then tile over the paint as long as it looks sound. and not easy to peel off. If the drywall is raw, you shouldn't tape the drywall seams because regular all-purpose drywall compound will re-soften from the water in the mortar and you may lose a tile there. If you've already taped the seams, or are concerned about the surface, still no worries. Get a small container of Custom Building Products' Redgard and put two coats where you will be tiling. Let the first coat dry (it goes from pink to red when it's dry) and then apply the second. After it turns red you're ready to tile. Tile directly over the Redgard. This stuff is both a crack isolation membrane and a waterproofer (if applied thick enough) but in this case you're just using it to create a nice tile-friendly surface. If you like overkilling things, you could apply Kerdi or Wonderboard to the ceiling but it's really not needed.
Lots of people promote the suction cup idea of building the ridge. I don't like it. First of all, it isn't necessary. Trowel the thinset onto the ceiling using the proper size square notched trowel for your tile size. Then - very important - flat skim the back of EACH TILE before you set it. This takes a little longer, but you will get great coverage and nothing will go anywhere. Pull off a tile every so often and make sure that you're getting good coverage. If you have good coverage, then you automatically have a vacuum there. If you're taking your time, and the thinset you've troweled out starts to dry out (doesn't feel sticky to the touch) scrape it off and put on some fresh. You will have no problems.
The other problem with the thinset ridge/suction cup plan is that you get poor coverage. If you lose a ceiling tile, it is likely to crack or chip whatever it falls on. Sometimes that's a shower floor tile, sometimes it is a $2500 cast iron bathtub. Any type of adhesive bond joint (thinset is essentially an adhesive) depends on surface area bonded... so you want good coverage.
The worst part of the job is grouting. It's a mess... wear a hat and old clothes & shoes. Grout falls like mad. Pesky gravity.