Yeah, it has gotten worse over the years. I suppose in most cases they do probably make up more than 25% of the total page, and easily if you count just above the fold. And yet, for me anyways, they aren't near as obnoxious as the ads on Answers.com and other sites that put banner ads on every corner of the page, and insert text ads between every paragraph. So, I don't know. Maybe it's more than just how much real estate you use, but I think my original point still stands.
http://news.ycombinator.com/item?id=4476508