The tag most appropiate for the banner ad was an “aside” tag.
I would disagree. <aside> is kinda like a sidebar: the content inside it is supposed to be vaguely related to the main content on that page, but can stand alone on its own.
Now, people do tend to put ads in their sidebars, and if you have a sidebar with aside-type info AND a banner, I think you could use aside… but what is the ad about? Where does clicking it take you? I think you could only argue <aside> if it took you elsewhere on your own site, to a page with relevant and at least mildly-related content to whatever was on the page where the ad is.
Even with all the new tags, if I’m grouping stuff simply so I can style it (grouping not-necessarily-related content so I can float them all together for instance), I’ll use a plain old <div>, which was created exactly for this purpose: grouping stuff no matter if it’s related content or not.