I am experimenting with DIV elements and plan to eventually convert my
existing page, designed with tables, to DIV.

Right now, my tables have essentially three cells in a single row. The
left and right cells are "hard-coded" to 150 pixels. The middle cell
is undefined -- thus, it expands based on the browser width.

I would like to replicate this expandability to a DIV structure. I'm
using absolute positioning to put the left "cell" (DIV region) at
LEFT:0, the right region at LEFT:550, and both are 150 pixels wide.

My question: How do I indicate in my CSS code that the middle cell
should expand to fill the width remaining?  (If these were frames, I'd
use a  *  symbol to indicate expansion -- as in '150,*,150')

Help!

width=100% ?

But it's not truly 100%... it's the balance of whatever's leftover.

Float one div left.  Float one div right.  The main content (not in a
div - just in the body) will render as a middle frame.  Now, the
middle frame will be the full width ("under" the floating divs), so
you need to fix that.  If you look at the CSS formatting model, you'll
see that it's padding, not margin, that you want to change.

DIV.left {float:left}
DIV.right {float:right}
BODY {padding-left: 160px; padding-right: 160px}

-Haversian-ga

Might as well set the width in CSS too:

DIV.left {float:left; width: 150px}
DIV.right {float:right; width: 150px}

How about using some javascript to do your task, setting your div
width based on your window size minus your left and right column
width.  If you want to get creative you could pass the column widths
in as params, instead of hard coding them like I did.

function doThis() {
  // IE
  if(!document.layers){
    // set div with id "divName" to have width of window - 160*2 
    document.all.divName.style.width = document.body.clientWidth - (160*2);

    // you may need to use .offsetWidth instead of .width and/or .clientWidth
  }
  // netscape
  if(document.layers){
    document.divName.width = window.innerWidth - (160*2)
  }
}

SitePoint has a three-column CSS layout tutorial:
   http://www.sitepoint.com/article/1213