What's PSA? It's Public Service Ads, Google Adsense PSA show some ads for public service, including US Election 2008 nowadays, Yeah you can easily find Google Adsense PSA about Election 2008 on many blogs/sites today.
The most important point; PSA is FREE, it means when anybody click the ads, the publisher got nothing. What's the reasons they (blogs with PSA) show PSA? there many reason guys ;
1. Non supported language
Yeah the blogs/sites have no English content at all, even they have some English contents, no one can guarantee that Adsense commercial ads shown.
2.Not related with any content
If your blog/site didn't have content that related with any Adsense ads, PSA will show up on your blog/site.
3.Your blog was banned
When your blog (with Adsense on it) 'fight' with Google Adsense Policy, your blog won't indexed on SERP (Search Engine Result Page) then your ads will change with PSA.
Actually to be an succeed Adsense publisher means you must do hard work on learn SEO, whoever are your prophecy, you must be a webmaster. Trust me !!