> everybody else.  Comment spam -- at least the kind that I keep getting 
> hit with -- isn't likely to pollute search results for anything other 
> than the keywords its targeting, which, frankly, are a small set.  The 

I assumed that at least part of the page ranking was general popularity,
and didn't require keyword matches on the referring page.

However, the general pollution of the document stresses that it is not
the links that need to be marked as unreliable, but the whole of the 
contributed content (at least until vetted by the site owner).  By
attacking links, Google are dealing with a problem with a specific 
mechanims in their page scoring, rather than thinking about how to
properly mark the quality of the whole resource.

A lot of the discussion here has been about alternative attributes
on A elements, but I think the real answer has to be applied to all
unvetted third party text.  Different search engines may have different
policies:  some may completely ignore the material; some may ignore 
links and de-rate keywords;  some, if they associate keywords with the
links, may limit the scope of keywords in third party material to the
links in the same material, avoiding pollution of other links with
inappropriate keywords.