I'm gonna have to thank you profusely for this answer. I did indeed have my IE browser set to "quirks" mode, though I honestly know nothing about it, and when I changed over everything came up correctly. I was disconcerted, though, to see the same bizarre display variations when checking the site in netrenderer.com, as well as on a lab computer yesterday -- all the same issues were present.
Am I to assume all those computers / renderers were set to quirks mode and thus rendering incorrectly? And what does that mean for the average IE user? Without messing with their settings, are they seeing the site as I intended, or is quirks mode something that comes up by default?
FYI, I'm a mac user, so IE testing involves either pulling out an old macbook running vmware, or using netrenderer. Not very familiar with how IE is set up for most people.
Thanks again, Andrew.