If you haven't watched the last episode (aired 4/13, 2014), "The Lion and the Rose", stop and catch up.
I've watched the last scene twice, the second time in slow motion, and I'm working on theories. I haven't read all the books yet (I'm still reading the first one), so my ideas are based only on the aired episodes.
First of all, in one of the previous episodes Pycelle said that poison is the preferred weapon of women, cravens, and eunuchs.
We have one eunuch, Varys, and a few women who were very close to the crime scene.
I've noticed that Margaery Tyrell didn't taste the pie. Joffrey clears his throat just after eating the first mouthful of pie, and only then he asks for the wine, complaining because the pie is too dry.
In her conversation with Sansa Stark, Lady Olenna Tyrell said "Killing a man at a wedding ... What sort of monster would do such a thing? As if men need more reasons to fear marriage." Those words sound so ominous after the end of the episode. While the others are still processing what's happening, Lady Olenna is the first one who yells "Help the poor boy! Idiots, help your king!" Doesn't she seem too ready to react?
What are your theories?
If you want to test other ideas and characters, this post will give you some hints.