Possible suspects:
Prince of Dorne - Motive: he came to town with revenge on his mind
Sircee (sp?) - Don't put anything past her. Her new fondness of the old Meister could bring access to the poison, whether through the wine or the pie. Motive: The way I'm thinking, she is still the Queen Regent, right? That's really all she ever wanted. Even she knew Joeffry was beyond reproach and was standing in the way of her power.
Tyrian - I'm going to throw him in here because he has threatened the king several times, but I really doubt he did it.
Sansa - If anyone had a motive to kill Joeffry, it was her. I don't think she has the constitution for such a thing, but she definately wanted to see him dead.
The Fool -- After the public humuliation and demoting, he certainly had a motive. I can't remember if it was in the episode or in the previews to next week, but he is trying to usher off Sansa amidst all the commotion, so even if he didn't do it, he was probably involved in some way.
Hell, just about everyone had a reason to kill Joeffry. My money is on Sircee (sp?) though. She has the most to gain and she is one evil bitch, so killing her own son doesn't seem to far out of character ... especially when she can frame her brother (Tyrian) in the process. That said, finding out anyone who did it won't shock me.
