The reason you are seeing a public service ads because google doesn't know what ads to serve...this is based on your meta tags, your url folder tree...modify those to get what you want. It is really easy...
You may also need more content on your page, if Google deems the site to be of very low quality it may not serve ads up. Also, some illicit content sites may never see the ads. I have had ads show up as public service at first and then change to normal ads after an hour or so too.
google also displays pulic ads when your content it is considered by spiders that it is inadequate for google ads. an example would be if it consider it too sexy
If you have multiple sites it might be because one of the sites you have are performing poorly...this happened to a friend of mine and when he removed adsense from one of the sites that was performing poorly his sites went back to normal