If you work in an abortion clinic for any length of time, as I did, you get used to discussing procedures for responding to things like bomb threats. These and other threats of violence happen, though maybe not often enough to say "regularly."
The discussions of how one would respond to them, and which exit you evacuate through, and which law enforcement agency you call at which point, are usually abstract; something you learn about upon starting employment, and review either annually thereafter or (if your sense of humor or taste in discussion topics are morbid) with your coworkers over drinks. In the three years I worked at the Austin Women's Health Center we dealt with vague references to violence, but never the real thing.
A few days ago someone actually left a bomb at the clinic. It apparently was real, and was loaded with over two pounds of nails. It was safely detonated by police, but, had it not been, it could have killed people.
A man was arrested yesterday. He doesn't have any connections to anti-abortion or domestic terrorism groups that are known; he sounds more like a fuck-up. He works a few part time jobs, and was paroled in August 2005 after serving less than three years of concurrent 15 year sentences for burglary. He's now charged with three federal crimes: using a weapon of mass destruction; attempting to damage a building used in interstate commerce; and attempting to damage a building that provides abortions.
If convicted on the first charge alone he could go to prison for the rest of his life. If he did it, that seems like a fine place for him.