Just noticed (again) an ad for SO on SO.
Seems the ad-scheduling should/could be a teensy bit more intelligent. ;-)
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...
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.