The book didn't go into the Horcrux process very much, but it does seem like something that is a complicted piece of magic and not something that could be easily or accidentally done. It also sounds like the horcrux would be carefully chosen to be something the maker felt he/she would have complete control of (or at least access to) and the item was special to them in some way. The things we already know about and where they are hidden have rather personal meanings to LV.
Thinking along those lines, making a horcrux out of something like a potion that who knows who would drink doesn't make any sense. The potion in the cave was well protected, but LV would have no control over who drank it. And, if someone did drink it, would he even know who drank it so he could find them again? I think not, since he does not appear to know that DD already destroyed some of the horcruxes. Besides that, the process of creating a horcrux seems to involve causing a death at the time of creation; not something that could be done with a potion sitting around waiting for someone to drink it.
And if you made someone your horcrux, what happens if they die, your horcrux goes with them? Or they make personal decisions that damage your soul in them?
I think there are too many problems with a living horcrux (other than Nagini), Harry, his scar, or DD being a horcrux or an accidental horcrux being made. One of the other things to think about is that in the things LV and the Death Eaters have said about Harry in all the books to date, is that LV doesn't understand why Harry didn't die when LV did the Avada Kadavra curse on him. Not something he would be thinking about if he had intended to make Harry a horcrux. He would have wanted Harry to live a long time. Maybe in the next book, we will find out that one of both of Harry's parents were killed to "seal" a horcrux that is connected with Griffendore (after all they lived in Godric's Glen), but I think it will turn out that they were just killed because they had a son born at the right time to fit the prophesy.
I think the locket is just what the note says it was, a substitute that RAB (whoever that turns out to be) put in the place of the actual horcrux locket when it was stolen. The mystery is going to be who took the real locket, whether or not it was actually destroyed and if not, where the locket is now.