The alleged sender of those ricin letters turns out to be a political-fringe letter-writer named Paul Kevin Curtis who has an interesting story to tell (see the last blockquote) (here's a longer version). He claims to have uncovered an organ theft ring at North Mississippi Medical Center and that a conspiracy of politicians, local media, and judges are trying to silence him. He is also into the chemtrails conspiracy theory.
About the ricin letters, there are four possibilities:
1. He did it and it was ricin.
The most straightforward answer raises the question of what made him decide to kill people. Motive is simple: he thought they were not paying attention to him. How does that become justification for killing politicians who have no interest in his story, and why now? He had been pushing the story for a long time.
If he did it, why would the dumbass sign the letter? He wanted publicity. He is not afraid of going to prison, and is probably bankrupt and has nothing left to lose.
Confirmation of this scenario will come from the FBI finding whether he has the materials and knowledge to make the stuff.
2. He did it and it wasn't ricin.
In this scenario, Curtis daubs the letters with castor oil to trigger the scanners and get himself noticed. He would have to know that there were scanners that would be triggered by what he was doing.
3. He didn't do it and it was ricin.
This is the X-Files scenario where he gets joe-jobbed by the CIA. Someone intercepted his mail, added the ricin, and resealed it, or they simply sent their own mail copying his recognizable writing style. The biggest of the plot holes in this story is that there is no reason for the smoking man to do this now when Curtis has been harmless for fifteen years.
4. He didn't do it and it wasn't ricin.
The innocent-mistake scenario raises the question: Then what the hell was it?
Edit April 22: The new theory is a joe-job by some griefer that Curtis met online. The new suspect is J. Everett "Dusty" Dutschke. I heard on TV that he also has a large web presence, but all I can find are news stories and one press release about music. We currently know nothing about him other than that Curtis's lawyer fingered him as somebody who Curtis was feuding with and who could have had the motive to frame Curtis for a crime.