Big companies' ad buyers are warming up to the likes of blogs, RSS and podcasts.

But analysts warn that marketers need to approach this typically user-generated content with care.

According to researchers at PQ Media, advertising spending on blogs, RSS and podcasts will reach nearly $50 million this year. As user-generated content matures, it is predicted to account for more than $750 million of advertising cash by 2010.

Analyst house JupiterResearch tells marketing professionals to keep their eye on blogs, where opinions that can harm brands can be easily and quickly circulated.

JupiterResearch also notes that because so few of us actually make our opinions known online, those who do--typically, young men--are exerting a disproportionate influence across the blogosphere.

About 23 percent of Europeans make their opinions known through blogs, said the analyst house. JupiterResearch advises businesses to use "buzz monitoring tools" to see how widely the thoughts of disgruntled customers have spread.

Jo Best of Silicon.com reported from London.