Forum Moderators: martinibuster
I know that initially only PSAs are displayed till the pages are atleast crawled by mediabot.
Mediabot doesn't (at least for me) crawl the site - it only goes to pages which have been specifically accessed by users, and doesn't follow any of links on that page.
So each page will show PSAs (or your alternate ads) the first time it is seen. Some time later (usually under half an hour) that page (not the links on it) will be read by Mediabot and then start showing targetted ads.
After a couple of months with Adsense, my site of 80,000+ pages has still got plenty of pages that are only now being seen for the first time. Not that it really matters, like most sites most of the traffic by volume is on a fairly limited number of pages.
<edit>Jenstar typed much the same thing quicker... :-) </edit>