div tags within a span tag / tableless css

I am trying to design a page using a tableless css layout.  Here is the problem: a modal panel opens up using ajax.  The layout is contained within the modal panel which renders as a span (the modal panel), no other choice here.  I am trying to create two separate cloumns within the modal panel, one on the left and the other on the right.  For some reason, whenever I attempt this the controls within the left and right divs go out in a straight line horizontally instead of vertically in a straight line, even if I set the widths for the divs.  It seems as if the div tag borders are not being honored.  I have tried both static and relative positioning with the same effect.  Could someone point me in the right direction to solving this problem?
here is a sample scenario:

<code>
     <span>
      <div id="divleft" class="position:relative;width:150px;float:left">
           label
           textbox
           label
           textbox
           etc.
      </div>
      <div id="divright" class="position:relative;width:450px;float:right">
           label
           textbox
           label
           textbox
           etc.
      </div>
     </span>
</code>