It is a fact of life that we do tend to have more faith in what we are familiar with. It makes sense, we are animals, wary of the unknown until it is proven. So a .Com is probably trusted more because it is familiar. Incidentally it is likely to be the extension that is most used in a particularly area/country. The constant repetition of the .Com or .co.uk or whatever (and it seems there is just one) in the mass media makes a particular extension the default or usual one. We like usual its reassuring and hence trusted more. The next question then is, do they notice an extension anyway. I would say mostly yes, but others who may have changed their extension in an adwords campaign or where they could compare would know.
[edited by: Durham_e at 4:28 am (utc) on Feb. 18, 2008]