My experience has been that I can seem to get away with SOME duplicate content and it doesn't seem to have a big effect. I had a dentist client and we used the exact same site with the exact same information on it except the domain name and geo-targeted keywords changed. The first 3 sites all ranked well for each of their geo-targeted keywords. So... I put up 12 of these sites... and 9 of them disappeared!!! We went back and re-wrote them, with the same basic information but a pretty good rewrite, and all of them came back within a few weeks.
So, from that one small experience I have come to believe that there is a certain amount of duplicate content that google will accept, but once the threshold is triggered, there could be trouble!
I'm assuming the threshold is different for age of domain, niche, keywords, etc... but I do believe there is a duplicate content "filter", if not a penalty.