More on link types and the Semantic Web
One problem with the rel tag is that it does not support name/value pairs. For example, I might want a <a href=...> tag to have an attribute for how much I agree with the linked page. Something like this: <a href=... agreewith="3" ...> where the numeric constant might be assumed to be in the range [-10,10]. Still, even rel tag values like disagreewith, agreewith, etc. would be useful.
I think that eventually the Semantic Web will, in some form, catch on but I think its eventual success will come from small grass-roots efforts that are simple to implement and become de-facto standards if they become widely used. I believe that the most important semantics for linked web sites is what the level of trust or agreement is. For example, if a very large number of people link to a site that they believe to contain incorrect information, the dubious site's page rank will be high and the dubious site may be taken to contain accurate information. Widespread use of trust attributes on links would create new possibilities and opportunities for people writing software agents.