A strategy for dealing with PageRank spam on blogs and wikis.  Google suggests adding 
    rel="nofollow"
to all user-submitted links.  Spiders would notice the attribute and not grant additional pagerank based on the presence of the link.  See http://googleblog.blogspot.com/2005/01/preventing-comment-spam.html for details.

SunirShah has a thorough analysis of the shortcomings of this proposal. See the SisterSite link to MeatballWiki below.

if it is possible to only rel="nofollow" some links it may be useful i.e. only nofollow links not on a WhiteList will address to a large extend Sunir's last point although there is still the not noticing problem

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CategoryWikiSpam