I thought only the big theater version took arbitrary speaker positions, and the home theater version used premixed channels. But now I can't remember where I read that, so I could be wrong.
My Atmos receiver required calibration of the speaker placement from multiple locations within my listening area. I'm running a 7.1.4 setup (7 surrounds, 1 subwoofer, and 4 overhead channels). I've done a lot of reading about Atmos as well; I'm fairly certain it isn't premixed channels. My ears also agree - Atmos feels incredibly immersive.
It doesn't look like that's actually the case, as Dolby's promo page for "Dolby Atmos for the home" calls out having "up to 128 simultaneous independent audio objects in a mix":
I think most home theater systems end up mixing down to Dolby surround channels practice, of course. My receiver supports Atmos, but I still only have five speakers. :)
