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I would suggest giving your site a once-over with this [webaim.org]. I need to install it on one of my Windows VMs because I just found out my nice new Intel Mac won't work with it.
I would certainly recommend a scrolling <div> over an <iframe>.
However, where they become an issue is for people with limited motor control. Quadrapalegics, stroke victims, etc.
That goes for any scrolling elements on the page, not just <div> elements.
That also goes for elements that you hide on the page, like print headers and layout elements that are not being used. A blind reader will often find all of them, and dutifully read them off to the poor hapless victim upon whom you are inflicting your page.
All that being said, you can't cripple your site to serve those with additional accessibility needs, or you wind up with a butt-ugly site that the vast majority of your users will find unattractive. I tend to make sure that my sites validate WAI Priority 3, and usually ignore many of the warnings if they will interfere with the presentation of my pages. That's more than 99% of the sites out there.