Just noticed (again) an ad for SO on SO.
Seems the ad-scheduling should/could be a teensy bit more intelligent... ;-)
I don't really think there are any ads for SO on SO. There may be ads for SO Careers, other StackExchange sites, etc, but I have never seen an actual ad for SO.
There is one that says: "Want to reach top devs? There's a site for that.", but that is one for advertising on SO.
About the timing issue. I would expect to see the SO-related ads more often that any other, especially if publishers put limits on where their ads are shown, how many people, etc.
Even if there was an ad for Stack Overflow (which I haven't seen), what's wrong with that? It would be an intelligent bit of self-promotion. Only a small percentage of people reaching the site know about all its advantages. Stickiness.
Haven't you ever seen an ad for the television station you are watching? A "subscription pitch" for the magazine you are browsing? Wal-mart is full of advertisements about their lower prices. These cookies I just opened have a card in it about how fresh they are. So...