Awesome Mixed-Origins Storyline.
This is, in my opinion, THE best Joker origin story. In this issue we see Jack as already having assumed his Jokerpersonality, long before his inevitable chemical bath. He is already obsessed with Batman as his new playmate & toy.
We also get to see a nice little conversation between Batman & pre-Scarecrow Johathn Crane, who are meeting for the first time. You have to love the irony of Batman having once relied on Scarecrows studies to help him take down criminals, only to have the man become one himslef.
Afterwards, the reader's suspicions from a few pages are confirmed: Batman is responsible for the Joker's smile...at least the inhuman length of it. The chemicals are still to blame for the nerve damage. Of course you could easily excuse the whole nerve damage bit, seeing as the Joker has frowned plenty of times. The cuts just make a more logical smile. It also goes better with the Joker's Batman RIP smile, which has been stitched where the the cuts would have been. This could also close the plot-hole of the Joker's scars in the Dark Knight movie.
The last page shows an interesting turn: Has Batman put out a hit on Jack? (Evil to fight evil).