I just got through watching the last 20 minutes, and 3lite's right. If you pay enough attention, it all but spells it out for us. The Tyrells are obviously the fulcrum on which the whole plot turned. Sansa likely had a part, since she had come to trust the Tyrells over the last season, and had more reasons than most for wanting to see Joffrey dead. Sir Dontos, feeling both indebted to Sansa for saving his life, and not being too big a fan of Joffrey for making him a literal and figurative fool, helped her out in some way, probably at her request.
The vaguely middle eastern couple who's name I can't recall at the moment, are probably a red herring. I'm thinking they might end up doing something to one of the Lannisters (I'm thinking they'll eventually off Tywin), but a poisoning is too subtle for them. If they killed Joffrey, they'd want the Lannisters to know it's them who did it.
...or they could've been the ones who provided the poison. Who knows?
Though it doesn't spell it out directly, that final scene does make it fairly obvious who's involved.
edit: I should also add that the Tyrells did have something to gain from all of this, and they didn't kill Joffrey simply as an act of altruism, stopping a cruel tyrant before he fully hit his stride. Margery is now, by law, a Baratheon. Though she won't take the throne directly, she will be the queen regent until Joffrey's younger brother marries. Though I doubt Cersei will willingly give up her hold on power, Margery, and thus the Tyrells, have a legitimate claim to the throne.
Rez, you're getting warm but I don't think you got the full picture just yet. You man want to rewatch the entire episode again if you have the time to do some more detective work.
Also, a little backstory on the Martell couple. Oberyn Martell, or the Red Viper as they call him, is very knowledgeable in the art of poison. I have no idea why the show did not convey this fact is beyond me. The reason they even call him the Red Viper is because he poisoned a man in a bout. Oberyn has actually forged a few chain links (the equivalent of a few college degrees) and spent a lot of time across the narrow seas studying lots of different cultures and poison/spices.
During the episode, you notice that Oberyn gives Loras the sexy eyes. What is strange about that is in the books, the Martells and Tyrells do not see eye to eye. Long ago, Willas Tyrell (Marg and Loras's older brother) got accidentally crippled in a jousting tourney against Oberyn. Ever since then, there has been tension amongst the families.
Long story short, the Lannisters aren't the only ones who don't want Oberyn at that wedding lol.