I've just done a 'stolen content audit' on a site and (as well as entire pages being ripped off) found that a number of websites are stealing certain paragraphs from the home page too.
I can prove the theft but I don't know if you can start proceedings for just one paragraph.
The best information I could find was a statement on Chilling Effects that a word or phrase can't be considered copyright.
Does anyone know of anything official that indicates where the cut off point might be?