Trouble with content overlapping horizontal form using responsive Twitter Bootstrap

I have spent more time than I ever care to admit trying to fix this stupid bug. I have a form that displays to the left of some content. It looks okay on a wide and narrow screen, but on a tablet width (it overlays between 770 and 950 or so) the content to the right overlaps the form fields.

Am I missing something in my markup to use Twitter Bootstrap properly or do I need to add some custom styles using breakpoints to fix it? It appears that the fixed pixel width is causing the issue defined in bootstrap.css. Shouldn't these width properties use % since I'm using a fluid responsive layout?

<div class="container-narrow">
  <div class="content">
    <div class="row-fluid">
      <div class="span5">
        <form class="form-horizontal" id="applies-Step1-form" action="/" method="post">
          <div class="control-group">
            <label class="control-label required" for="Step1_email">Email <span class="required">*</span></label>
            <div class="controls">
              <input size="60" maxlength="255" name="Step1[email]" id="Step1_email" type="text" />
            </div>
          </div>

          <div class="control-group">
            <label class="control-label required" for="Step1_home_zip">Home Zip <span class="required">*</span></label>
            <div class="controls">
              <input size="5" maxlength="5" name="Step1[home_zip]" id="Step1_home_zip" type="text" />
            </div>
          </div>
        </form>
      </div>

      <div class="span7">
        <div class="promo-btm">
          <h2>Heading 2</h2>
          <ul class="checks">
            <li>Bullet One</li>
            <li>Bullet Two</li>
          </ul>