Sites seem to experiment with them and then turn them off after their traffic drops sufficiently.

E.g. washingtonpost.com had one, but stopped. Forbes had one, then stopped. Ars Technica tried one for 12 hours, and stopped.

The CW (www.cwtv.com) has one that pops up a few seconds after you start watching a television show that they offer you.

The detect-and-plea approach sticks around longer: The Atlantic, The Guardian and Mother Jones all have those.

Thanks. I've seen "pleas" on a couple sites but it wasn't a matter of detection; it was just part the default page served which they'd overwrite when/if their advert scripts ran. Without javascript, cookies, or third-party stuff loading, all you get is the default html.