I have my own ideas about the most common reasons that some sites get PSA adverts when they're part of the AdSense program, but let's start with what Google itself says in the AdSense help area (here's the link -- Why am I getting PSA ads? -- but you need to be logged in to your AdSense account before you can see the page):

• Our system has not yet crawled all the pages of your website.
• Our crawler can't reach your site.
• Your page may contain sensitive content for which relevant paying ads will not be displayed.
• Your account may be disapproved or awaiting review.
• Your site has restricted access using a robots.txt exclusion.
• Your website is using session IDs in the URL.
• Your website is using frames.
• Your ad unit is set to display image ads only.
• Your web page may not contain enough content.
• The AdSense ad code was placed within an IFRAME.
• Your web pages are behind a login.
• There are too many URLs added to your account filter list.
• Your site content is primarily in an unsupported language.
• Your ad code has been modified.
• Your page contains a refresh tag.

Two important notes to also be aware of:

1. The Google crawler indexes by URL and therefore considers http://site.com and http://www.site.com different URLs. Because their crawler will visit these URLs separately, you may notice ads appear differently on these pages.
2. The Google crawlers are unable to derive meaning from any of the following:
   • audio and video files (.wma, .mpeg, .mov)
   • mp3 files (.mp3)
   • images (.jpeg, .bmp)
   • Macromedia Flash movies
   • Java Applets

The most common causes of PSA ads in my own experience are pages that haven't yet been indexed (though loading the page a few times seems to quickly teach the AdSense system that the page exists and gives it time to spider and analyze that page for content) or pages that have content that's marked as being questionable or inappropriate.

There are plenty of topics that you should avoid, but honestly, most of them are unlikely to be a topic of your page, site or weblog anyway.

One more thought: log in to your Google AdSense area, then click on Reports --> Site Diagnostics and make sure that there aren't any problems with your site. Here's what you should see:

If you still can't figure out what's going on, don't forget that you can always email the AdSense support team and ask for advice. I have emailed them a number of times and found that they're quite responsive, though it can take a few days...

Good luck figuring this out!