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