<div id="container">
   <div id="header"></div>
   <div id="body"></div>
   <div id="footer"></div>
</div>

=========================================================================

html, body 
{
   margin:0;
   padding:0;
   height:100%;
}

#container 
{
   min-height:100%;
   position:relative;
}

#header 
{
   background:#ff0;
   padding:10px;
}

#body 
{
   padding:10px;
   padding-bottom:60px;   /* Height of the footer */
}

#footer 
{
   position:absolute;
   bottom:0;
   width:100%;
   height:60px;   /* Height of the footer */
   background:#6cf;
}

And one simple CSS rule for IE 6 and 5.5: !!! conditioneel

#container 
{
   height:100%;
}

Footer at bottom

HTML