Huzzah, there’s a new Staff Report here: http://www.straightdope.com/columns/read/2993 … to be posted on the website on April 26, 2011.
The question came from a thread in the “General Questions” forum, back in 2009, which is found here: Can I do a citizen's arrest on a trespasser? - Factual Questions - Straight Dope Message Board
Since that’s a zombie thread, I’ve closed it and suggest that folks post responses in this newer thread.
Just out of curiosity, did two years of research actually go into this?
No, the Staff Report was written some time ago. There’s always been a gap between the time the Staff Report is written and the time it’s posted on the website – they’re not done week-by-week, they’re done well in advance. The editing process has just been a bit [del]wonky[/del] erratic over the last year.
I’ve made citizens arrests. Lots of them. At least one for criminal tresspass. If anyone has any questions about it…
Please note that there’s another thread about this Staff Report: Re: Jehovah's witnesses at the door - Cecil's Columns/Staff Reports - Straight Dope Message Board
Since this one deals with the general notion of citizen’s arrest for trespassing, and that one deals specifically with Witnessing, I’m gonna leave them both open. Two different topics, although related to the same staff report.
The link to “Can I legally prevent people from putting flyers under my door or on my windshield?” is broken.
We type really slow. This post took me over six hours.
I love you pravnik - just thought you should know
Thank you! Love right back to you. 
I cannot think of citizens arrests without automatically recalling that old Andy Griffith episode with Gomer Pyle making one on someone making an illegal turn, shouting “Citizen’s array-est! Citizen’s array-est!” endlessly.