Q:

div with dynamic min-height based on browser window height

I have three div elements: one as a header, one as a footer, and a center content div.  the div in the center needs to expand automatically with content, but I would like a min-height such that the bottom div always at least reaches the bottom of the window, but is not fixed there on longer pages.
For example:
<div id="a" style="height: 200px;">
  <p>This div should always remain at the top of the page content and should scroll with it.</p>
</div>
<div id="b">
  <p>This is the div in question. On longer pages, this div needs to behave normally (i.e. expand to fit the content and scroll with the entire page). On shorter pages, this div needs to expand beyond its content to a height such that div c will reach the bottom of the viewport, regardless of monitor resolution or window size.
</div>
<div id="c" style="height: 100px;">
  <p>This div needs to remain at the bottom of the page's content, and scroll with it on longer pages, but on shorter pages, needs to reach the bottom of the browser window, regardless of monitor resolution or window size.</p>
</div>

A:

Just look for my solution on jsfiddle, it is based on csslayout

html,
body {
  margin: 0;
  padding: 0;
  height: 100%; /* needed for container min-height */
}
div#container {
  position: relative; /* needed for footer positioning*/
  height: auto !important; /* real browsers */
  min-height: 100%; /* real browsers */
}
div#header {
  padding: 1em;
  background: #efe;
}
div#content {
  /* padding:1em 1em 5em; *//* bottom padding for footer */
}
div#footer {
  position: absolute;
  width: 100%;
  bottom: 0; /* stick to bottom */
  background: #ddd;
}
<div id="container">

  <div id="header">header</div>

  <div id="content">
    content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>content<br/>
  </div>

  <div id="footer">
    footer
  </div>
</div>

A:

I found this courtesy of ryanfait.com. It's actually remarkably simple.
In order to float a footer to the bottom of the page when content is shorter than window-height, or at the bottom of the content when it is longer than window-height, utilize the following code:
Basic HTML structure:
<div id="content">
  Place your content here.
  <div id="push"></div>
</div>
<div id="footer">
  Place your footer information here.
</footer>

Note: Nothing should be placed outside the '#content' and '#footer' divs unless it is absolutely positioned.
Note: Nothing should be placed inside the '#push' div as it will be hidden.
And the CSS:
* {
  margin: 0;
}
html, body {
  height: 100%;
}
#content {
  min-height: 100%;
  height: auto !important; /*min-height hack*/
  height: 100%;            /*min-height hack*/
  margin-bottom: -4em;     /*Negates #push on longer pages*/
}
#footer, #push {
  height: 4em;
}

To make headers or footers span the width of a page, you must absolutely position the header. 
Note: If you add a page-width header, I found it necessary to add an extra wrapper div to #content. The outer div controls horizontal spacing while the inner div controls vertical spacing. I was required to do this because I found that 'min-height:' works only on the body of an element and adds padding to the height.
*Edit: missing semicolon

A:

If #top and #bottom have fixed heights, you can use:
#top {
    position: absolute;
    top: 0;
    height: 200px;
}
#bottom {
    position: absolute;
    bottom: 0;
    height: 100px;
}
#central {
    margin-top: 200px;
    margin-bot: 100px;
}

update
If you want #central to stretch down, you could:

Fake it with a background on parent;
Use CSS3's (not widely supported, most likely) calc();
Or maybe use javascript to dynamically add min-height.

With calc():
#central {
    min-height: calc(100% - 300px);
}

With jQuery it could be something like:
$(document).ready(function() {
  var desiredHeight = $("body").height() - $("top").height() - $("bot").height();
  $("#central").css("min-height", desiredHeight );
});

