Online video ad spending in the United States will keep growing by 22.5 percent this year. However, the increase has slowed down since 2006, according to estimates by AccuStream iMedia Research in January 2009.

The growth level last year was 36 percent, but it would rise again in 2010 to 28.2 percent, according to eMarketer.

Overall spending accounted for $2.1 billion in 2008, while CPMs on premium preroll videos averaged $35 in 2008.

AccuStream's estimates are far higher than those from other researchers. For example, according to eMarketer, online video ad spending reached $587 million in 2008. However, AccuStream counts all online-video-related infrastructure in its estimates.

eMarketer predictions also show online video ad spending growth would cool from from 81 percent in 2008 to 44.9 percent this year, and will bounce again in 2010.