Hello,

I am not a Google Analytics expert, but from my understanding:

Google Analytics will only record a visit when the google tracking code is triggered. Hence, if a page has not load properly you may see that you got a click from the search engine but no visits. Secondly, Google Analytics recently added Bing as a search engine, hence depending on the time frame you are looking at, Bing could show up as one of your referral sites and not as an engine. There could also be other reasons that I am not aware of.

In regards to bounces, I believe Google Analytics, will record a visit as 100% bounce if the user once visited that one page and left.  If that is true, the bounce rate is a good indicator on the interest of the user to your site.

You may of done this already, but it is good idea for advertier's to make sure that their webpages works will all the avaliable browers. For example, if you page is not Safari compatible then when a user that arrives at your site with a Safari browser through some media channel, your page will not load correct  and not record that visit in Google Analytics. (At the min. I will check compatibility with IE and Firefox)

Hope this help answer your questions.

Jason