At last! a definition of semantic publishing I can understand and that tells me why I might want to bother:
anything that enhances the meaning of a published journal article, facilitates its automated discovery, enables its linking to semantically related articles, provides access to data within the article in actionable form, or facilitates integration of data between papers. Among other things, it involves enriching the article with appropriate metadata that are amenable to automated processing and analysis, allowing enhanced verifiability of published information and providing the capacity for automated discovery and summarization.
From David Shotton’s excellent article Semantic publishing: the coming revolution in scientific journal publishing, in the April Learned Publishing doi:10.1087/2009202