I'm not finding a lot of real facts here, just a lot of heated blog posts. Did that reporter get arrested last night? Or was he just expelled from the building? Security guards don't arrest people. Did he just ask a single question, or was he badgering and interfering? Did he calmy leave when told to, or did he make a big stink? Did he actually have a press pass to this event, or did he barge his way in? Some accounts seem to hint that he might have been deliberately trying to create an incident.
Update: I've watched the video from David Weigel, and they aren't security guards, they look like genuine cops. But it also looks like he was merely escorted from the building for badgering and blocking behavior. I also note that all the news stories I've seen are press releases from biased sources.