eMarketer forecasts that paid search ad spending in the U.S. will grow 26.2 percent in 2006, seven percentage points less than last year's 33.2 percent gain, and the 174.3 percent gain in 2003. Growth of the maturing market will slow to percentages in the mid-teens through 2009, when paid search ad spend is forecast to reach $10 billion, according to eMarketer.

David Hallerman, senior analyst and author of eMarketer's report "Search Marketing I: Spending and Metrics," finds the slowing growth makes sense expected as the market matures: "Such maturity will include more vertical search for better targeting, and a sharp rise in local search, as both users and advertisers increasingly realize that the internet is the best place to make contact with any kind or size of business."

According to the report, paid search will continue to be dominant in online advertising, and by 2010 local search ad spending will constitute some 20 percent of all paid search spending, with the growth of local search outstripping national search through 2010.