According to Gartner the Semantic Web will be one of four technologies to drive the sevice driven IT recovery.
"...four new application/technologies (the semantic Web, the grid, enterprise performance management (EPM) and "net fabric") as being significant areas of innovation and opportunity taking shape behind the high-profile push toward Web services...Service and utility will be the overarching themes for new opportunity in the next five years as the IT industry undergoes its next paradigm shift."
Business Model for the Semantic Web "All you need to do is move up a thin layer of interoperability. Just as database systems suddenly became compatible by adopting a consistent relational model, so your unstructured data can also adopt a relational model, and get all the benefits you need to solve these problems.
The relational language for data on the Net is called RDF. When information from two sources is in RDF needs to be merged, you basically concatenate the files into one big file. When you want to extend a query on an RDF file to include constraints from another, you just write it in."
Lists of Semantic Web companies: Semantic Web Business SIG and WS Index.