There are a number of standards for supporting 'tags' in RSS including:
dc:subject RSS 2.0 'category' mod_taxonomy
How are we going to build a generic API that supports all systems currently in use?
Delicious
<taxo:topics> <rdf:Bag> <rdf:li resource="http://del.icio.us/tag/programming"/> </rdf:Bag> </taxo:topics>
Technorati
Technorati tags seems totally proprietary:
To contribute to this tag, just make a post to your blog about met and include a link to this page like so: <a href="http://technorati.com/tag/met" rel="tag">met</a>
Which is totally proprietary...
Ideal HTML specification
http://www.peerfear.org/rss/permalink/2005/01/18/TechnoratiTagAPIIsBroken/
Interface
onTag( String tagname, String uri )