I think there's a lot to fire up crazy people in general right now politically, but Obama's race is certainly novel and likely powerful for racists.
I'm not trying to suggest that people shouldn't be worried about right wing extremist violent craziness because there's also left wing extremist violent craziness, but I'm not sure I agree that there are fewer left targets or that there's been less that threatens human health/life. Think about the WTO 1999 stuff in Seattle or check out this wiki entry on one of the animal rights groups. They were targeting occupied facilities with bombs
http://en.wikipedia.org/wiki/Revolut..._Cells_(RCALB)
So far their human casualties are pretty limited.
I agree that Ruby Ridge is no OKC, but I'm not sure it's as simple as intrusion on private land in its essence. And in terms of influencing or feeding right wing extremism, it was pretty significant. [I want to make clear that I'm not trying to blame the Weavers mainly or anything, but I just think that "intrusion on private land" kind of understates the whole deal.]
It would make sense that as the political power base moves away from one side, the crazies on the fringe of that side get crazier. And both the present stuff if it's a trend and the early 90s stuff to some degree seem to cluster following political transitions leftward. [ETA: Actually, here's more evidence that Ruby Ridge was a poor example to use earlier: It was actually too early to reflect a political shift leftward as a cause. The events are late in HW Bush's term, actually preceding Clinton's election.]
I'm just not sure that the incidents we're seeing now are part of real shift or trend (I have no idea, for instance, how many racially motivated killings there were in the last year before the FBI report.), and I certainly hope that they turn out not to be part of any big increase of violence of any kind.