41% of UK web users asked were unaware that Adwords links were paid-for adverts

"Research carried out by Bunnyfoot suggests that many people are unaware of the difference between paid and organic search listings, with 40% of web users unaware they were adverts.
While conducting a research project for an insurance sector client, Bunnyfoot discovered that 81% of users clicked on Google Adwords listings as opposed to natural search results.
Further investigation of this surprising bias revealed that 41 out of the 100 individuals tested did not know that Adwords were paid-for adverts, believing them instead to be the most authoritative links."
Source:  Econsultancy, 28th February 2013
Caution - 100 internet users is a small sample...