That is exactly how my mains are mounted. My choices were either be able to walk under the speakers, or put one on top of a bookcase scraping the ceiling and the other in a corner.
I cut a couple lengths of square tubing, welded a 4" square of .125 or so steel plate to one end of each, and mounted the speaker bracket to the other end by drilling and tapping some holes. When done, I filled the tubing with expanding foam in an effort to prevent resonance from the tubing.
Don't know if it did any good, but I havn't noticed any ill effects in almost 7 years of use. I drilled 4 holes in each plate and screwed them to the ceiling at a joist. I was able to position them exactly where I wanted out in the room away from walls with no concern for traffic or crap on the floor.
They're toed in and down of course. The soundstage is a bit high- but the overall result is surprisingly good, especially considering the alternative. I even ran a phantom center for a while, (I am right now as my center ch amp is in the shop)and it really isn't bad. The limitation is speaker size and weight, I would probably have m-22's by now except the height difference would put the tweeters pretty close to the ceiling. I've thought of mounting them upside down to place the tweeters closer to ear level, but havn't tried it. If you don't have people walking under them your options open up a little more. I can come up with pics if you want, but am too lazy to post them.
