In the United States, anonymous or pseudonymous works are protected for 120 years from date of creation--this addresses this issue, and would be a reasonable safety margin. Besides, if someone does come back and say, "WHOA, that's my father's estate, you can't publish it!" we'd what...take it down? 120 years is a very good place to draw the line.
As far as I know, a 120-year-clause does not exist in most european countries -- i could be wrong though. Anyway, just taking it down may be enough in some cases, but not all. Consider this case: someone finds an image on commons which is labeled PD, and decides to use it as a book cover. After the book has been published, it turns out that it's not PD at all, and the copyright holder sues the book's author/publisher for 100k$ and wins. The books publisher then turns on us (the foundation, the uploader, the reviewing admins) for compensation. Do we have legal safeguards against that case? I don't think we do. But it's a question for the lawyers, I guess.
Anyway, we had the "draw the line somewhere" discussion repeatedly before, in all length and width, with, as far as I know, no definite outcome, besides maybe that 200 years are probably enough. The first (afaik) thoughough discussion about this was actually started by me in early 2006: http://commons.wikimedia.org/wiki/Commons_talk:Licensing/Archive_1#Assume_PD_for_Images_older_than_100_years. See especially the commenty by Rama and David on the con side (and Arnomane and Histo on the pro side).
Basically: 100 years is not enough, and it's not quite clear what number of years *would* be enough. I heard about some rule of thumb being established at some point, but i don't know if it's actually written down somehwere.
-- Daniel