While looking for the answer to another question, I have stumbled upon a weird curiosity in the Pathfinder (and presumably 3.X as well) RAW.
Unconscious creatures are knocked out and helpless. Unconsciousness can result from having negative hit points (but not more than the creature's Constitution score), or from nonlethal damage in excess of current hit points.
The condition itself does not cause someone to automatically fall prone. Neither do the most common causes of becoming unconscious, such as Dying or Non-lethal damage, nor less common causes, such as the Brawler's Knockout ability.
Now I wonder if I have overlooked something somewhere, since PF rules are known to be all over the place (hello, touch spells).
While any sane GM™ would probably reply to the titular question along the lines of "Yes, of course unconscious creatures fall prone, they're effin' unconcious!", this is not subject of this question. I'm only interested in the rules here, for the sake of interest and for finding the RAW answer to the question linked above.