In Internet Explorer 8 ( When IE8 is not in Compatability Mode ), I have discovered that if you put a Form tag inside another Form tag, it will push floats out in ways that previous IE browsers do not.
This happenes to me a lot when I build .NET sites with .master pages. The master page will have a form runat='server' tag. So, if you put a form tag on one of your content pages, it will mess up your floats.
Good thing most IE8 users use Compatability Mode.