While I do believe that the general tone of the media is partly responsible for the shooting in Arizona, it is worth repeating again, and again, and again: There is no equivalency between the Right and the Left when it comes to incendiary rhetoric.
What Keith Olbermann and Rush Limbaugh do on a regular basis are not remotely comparable. Olbermann can be a bit of an ass sometimes, but his political views are in no way extreme. Olbermann does not encourage violence, does not condemn vast sections of the population for their ethnicity, laziness/voting a certain way etc, and does spend much of the time highlighting the plight of those without wealth or power. Rush Limbaugh on the other hand consistently riles up his audience against 'enemies' of hard working white people, makes overtly racist comments, and does his best to protect the rich and powerful from any type of scrutiny.
Cenk Ugur of The Young Turks demolishes the argument once and for all in this must watch segment (h/t Cesca):