in my experienceHowever, I can't imagine how many companies AOL must be pissing off by doing this, but then again, market penetration for Mozilla based browsers is barely in the range of what I would call "significant" so advertisers may not care. Stop and think about when AOL builds this into the AOL client, and keep in mind that uptake of new versions of the AOL client are robust to say the least (8.0 was released this fall, and I think it's already the most used version on the service).
When millions of user go to popup blocking, the popunder ad might become less viable. Unfortunately, according the C|Net "Netscape 7.01 comes with the pop-up filtering off by default."