I use Opera 9.01 as my default browser, and I was rather worried when the Google ads didn't show up in my browser, although they worked fine in the other browsers I use, IE6, IE7, and Firefox 1.5. I was about to post a frantic query to the support forum when I realised the reason. Opera has the ability to block certain content from being shown; at some stage I must have blocked an ad from Google and so it refused to show them. I checked, and, sure enough, a Google server showed up in the list of blocked sites.Panic over!
I also signed up for Google Analytics, which provides some detailed statistics on visitors to the site.