Last week I installed two stainless steel padeyes in my living room ceiling and successfully hung from them. The padeyes are 7/8" I.D. and are rated to over 2000 lbs. apiece. They are available from West Marine. About $20.00@.
I think that it is worth it to use these for two reasons. Being yacht hardware, they are very good looking in comparison to hardware store galvanised stuff. More importantly, though, is the fact that the padeye is attached to the joist via a base plate with two #14 screws ( I used 3" long
stainless screws) instead of a screw eye's 3/8" thread. This does not compromise the structural integrity of the joist as much, and distributes the load over a larger area.
The height of the ceiling is 8 1/2' and the span between padeyes is 22'. Both padeyes are screwed to the same joist, making it into a 2 x 8 compression member. No big mystery, then that there wasn't any deflection or creaking or anything when a load (my butt) was applied to the hammock.
Certainly, hanging from ceiling joists instead of wall studs puts much less stress on the frame of the house, but I don't think that it would be doable on a shorter span unless you had a compression pole to place in between the ends of the hammock.
I've slept several nights in this rig, and my back and shoulders definately feel better than when sleeping on my Tempur-pedic mattress.
Now I just have to install another set for my wife's hammock