A friend told me today that an advert wasn't showing on a site that they had paid to advertise on. To cut a long story short after several hours looking in googles cache and proxymouse we found some software on their server that invisibly blocked (by default) files from directorys like /banners etc and images called banner....gif etc. They are now asking the site to move the banners to the /images directory and rename them. The moral of the story is check the urls and filenames of all the image adverts on your site and other sites to make sure they are actually being seen by the maximum number of users.