Since AdSense launched, it has been a well known secret that a page with index.html?keyword would automatically display ads specifically for that keyword, regardless of whether mediabot had seen the page before or not.
For example, a page all about auto mechanics with www.domain.com/index.html?weight+loss would show weight loss ads, even if there was not a single word even remotely related to weight loss on the page those ads were "targeted" to.
There was a similar loophole when viewing local html with the AdSense code (testing pages on your computer before uploading them to the web). You could view your page, and the ads would automatically target to keywords in your directory or file names, again, regardless of what the on-page content was.
This loophole was often seen on directory sites & search engines (even though AdSense on search results is against the terms) because it would provide perfectly targeted ads every time, and show targeted ads every time - but it was also used to provide high-money keywords on pages that had absolutely nothing to do with that keyword, and particularly on auto-generated webpages.
Now, www.domain.com/index.html?weight+loss shows PSA ads, until the mediabot comes along, then it will start showing targeted ads - not targeted to the keyword, but targeted for the actual on-page content. AdSense viewed locally will now only show PSA ads.
I think this loophole fix has been a long time coming, and the writing was on the wall a few days ago that they were testing some sort of fix to this. It was great for seeing what ads a particular keyword or keyword combination would show in AdSense. But it was most often used to provide high money keywords, regardless of the page's subject, or to provide those keywords on auto-generated pages, where the only way to leave would be through the back button or an AdSense click, since no content on the page was usable.
And this also partially explains why there have been more PSAs reported lately, possibly from AdSense tweaking, or perhaps all index.html?keyword style of pages (even if the keyword wasn't a true keyword but another variable) were flagged to show PSAs until the mediabot could go back to each one and ensure they were properly targeted ads, and not targeted to the keyword instead.
This should be for the good of AdSense, and will help keep the Adwords advertisers happy, which AdSense should be doing, since it is those advertisers that fund the program to pay the publishers