Google is using semantics and has been for some time now. If you search for ~inkjet (the ~ tells Google to search semantically) you will see that Epson is semantically linked to the word inkjet.
I use this in two different way’s (really only on way) First I do check all my keywords for semantics. Knowing what words are semantically linked to the keyword is important, IMHO, when I optimize for the keyword. I always make sure that the semantic words appear on the page or I create specific content to optimize for this word also.
Also I think that semantics are important in supporting the theme of your site, and the theme is important in optimizing your site. A site with a clear theme, that is, links from and to other site within this theme, content to match the theme and keywords within the theme, will often land more targeted traffic and land higher in the ranks.
bufferzone: Could you please point me any documentaion on use of ~ in search requests
JamesR3: I understand mozzila make use of RDF, as do some news feed standards. Its a pity it has not taken off, need something to unify multiple xml formats currents in use, wheels turn slowly, much data still in CSV format and like.