I have noticed (though it has passed today) the same behavior as the original poster, and those times it is obvious that the ad server is holding up the rest of the page loading.

<shrug> If it keeps happening, I'll just have to use the junkbuster filter... But as of yet the ads here are not too intrusive (and at least for me they are most often USAirways, who does not need to advertise to me --> probably some of that targeted thingies they talk about. I should erase my cookies and see what I get)