Ezra Klein and others correctly point out that one can't infer causality from the direct inflammatory statements (e.g. the Sarah Palin's crosshairs map) to the shootings in Arizona yesterday, particularly if the shooter is mentally ill.
But, it's deeper than that. Causality is about the over-the-top Republican rhetoric that passes for policy argument these days. The health care bill isn't just policy we disagree with, it's unconstitutional. President Obama isn't just someone we disagree with and didn't vote for, he's illegitimate and has a secret Muslim agenda. Gay people are destroying American families. Democrats are socialists threatening our basic way of life. Throw in some violent imagery and veiled threats from Rush, Glenn Beck, Sharron Angle, etc.
Given that many people learn about current events only from Fox News and other conservative commentators that use this kind of "argument", it's absolutely no surprise that violent crazies emerge.
Republicans created and/or condoned a rhetorical environment which fosters violent acts. Thus, they share responsibility for the injuries and deaths which took place yesterday.
If you're a Republican or a conservative "independent", you should be deeply, deeply ashamed. You have blood on your hands now. As you always did, but now more than ever, you have a responsibility to object, both publically and in private with your like-minded citizens, to these absurd, offensive, and un-American ways of discussing our (legitimate!) political differences. And if you're a famous Republican for whatever reason, that goes tenfold for you.
Edit: Please note that the comments are not a space to post inane right-wing drivel. Such postings will be deleted. Thanks!