It appears that Wikipedia did it right with the entry for Advertising Agency Wiki

Lately, Search Engine Marketing (SEM) and Search Engine Optimization (SEO) firms have been classified by some as ‘agencies’ due to the fact that they are creating media and implementing media purchases of text based (or image based in some instances of search marketing) ads. This relatively young industry has been slow to adopt the term ‘agency’ however with the creation of ads (either text or image) and media purchases they do qualify technically as an ‘advertising agency’ as well as recent studies suggest that both SEO and SEM are set to outpace magazine spending in the next 3-5 years.

This is intresting for many of us,  because Ad Agency Review Consultants have yet to read the article.