***SPOILERS if you have not seen up to S4E3 Stop reading now***
More information has come to my attention...
The poison used on King Joffrey was called The Strangler.
He wasn't at the wedding was he? Little Finger had the necklace made and passed off as a House Hollard heirloom. Was Sansa an unknowing carrier of the poison because the gems of the necklace contained the poison? It's looking that way.
Little Finger smashes the gems with an object and tosses it on Dontos lifeless body. (Real gems won't break like that.)
Why isn't Sansa asking why he had the necklace made and then why he smashed it?
Hopefully Sansa got Ros' message about Little Finger she passed onto Shae at the wharf area in Season 3 (not to trust him). If Baelish is easy to discard Dontos he could certainly turn on Sansa pretty quickly.
In a way he kind of has turned on her by setting her up as the person who helped poison Joffrey.
If she ever leaves Baelish's company she could be killed.
If the necklace was used to deliver the poison, then this puts Lady Olenna from House Tyrell into the plot.
What kind of fair trial will Tyrion have with Tywin, Mace Tyrell and Oberyn Martell as judges? (Tyrion seems to think Mace Tyrell will do what Tywin wants. That leaves Lord Oberyn as the wild card.)
It seems that Tywin may have the same thing in mind when he asked Prince Oberyn to join his small council and a judge at Tyrion’s trial. He indicated that the Dornish suffered little from a Targaryen attack. Like the Godfather, Tywin is keeping his friends and family close (Tommen) and his enemies closer (Oberyn).
Tyrion seems to have a handle on the situation.
He's eliminated Sansa and Cersei as part of 'The ominous they' who framed him for killing Joffrey.
Is there anyone Tyrion can call as a witness that might testify on his behalf besides Sansa? Varys is already a witness for the Queen.
Is there a House behind Baelish or did he act on his own?
It's looking like House Tyrell is the one behind the killing along with Baelish, however as Robin has pointed out in this review:
it could just as easily have been Tywin deciding to get rid of Tyrion and Joffrey in one swift operation.
Game of Thrones: Threads at the TV Critic