I think I have found some solutions to your problem, although they are not all that pretty.
The first thing you could do is make your side-bars a fixed width and then back non-floated div 100 px to the right to make it seem as though you want:
Code:
<div style="float:left; width:100px;">abc</div>
<div style="border:1px solid #000000; margin-left: 102px">def</div>
Another thing you could do is 'fake' the no-border like the following:
Code:
<div style="border-left: 1px solid #FFFFFF; border-right: 1px solid #000000;
border-top: 1px solid #FFFFFF; border-bottom: 1px solid #000000;
float: left; margin: 0px;">abc</div>
<div style="border:1px solid #000000;">def<br>ghi</div>
This however, is not very pretty as it forces you to have the non-floated div to have more than one line of content, and it forces some strict rendering tricks to get it to work.
I can look into this further if you want, but that is what I came up with for now.
Thanks,
Ted Morse