It seems pretty well established in the books that splitting your soul requires murder (the supreme act of evil). I also don't think it's correct to say that there was no murder involved when the portion of Voldemort's soul attached itself to Harry. He had, after all, only just murdered both of Harry's parents. That's two acts of murder within a very short period prior to failing to kill Harry, and he had no doubt killed plenty of people without creating Horcruxes before then as well.
JKR: here is the thing: for convenience, I had Dumbledore say to Harry, "You were the Horcrux he never meant to make," but I think, by definition, a Horcrux has to be made intentionally. So because Voldemort never went through the grotesque process that I imagine creates a Horcrux with Harry, (SU: Mm-hm.) it was just that he had destabilized his soul so much that it split when he was hit by the backfiring curse. And so this part of it flies off, and attaches to the only living thing in the room. A part of it flees in the very-close-to-death limbo state that Voldemort then goes on and exists in. I suppose it's very close to being a Horcrux, but Harry did not become an evil object. He didn't have curses upon him that the other Horcruxes had. He himself was not contaminated by carrying this bit of parasitic soul.
The Leaky Cauldron, Transcript of Part 1 of PotterCast’s JK Rowling Interview
The specifics aren't known (yet), but I think we can say with a reasonable amount of certainty that every act of murder damages your soul. It may not be split in to two distinct parts yet - the Horcrux spell may do that, allowing you to extract the smaller portion and bind it to an object - but there's damage that has certainly weakened it.
The repeated process of damaging the soul leaves it full of cracks, weak points that simply need a sufficient amount of force to result in the entire thing shattering in to pieces. The rebounding Killing Curse provides that force: Voldemort's body is destroyed entirely, and his soul splits along at least one of those weak points.
Personally I imagine Voldemort, at the point just before trying to kill Harry, like a vase full of marbles. The rigid sides of the vase keep the marbles in place, but you can reach in and take them out individually, and put them elsewhere. However, if you take a hammer and shatter the vase the marbles go everywhere.
So, to sum up, murder is required to split your soul (or at least damage it enough that it can be split), but I don't think that act of murder would need to come immediately before the creation of a Horcrux, or even need to be committed with the intention of creating one.