You need to use a "trick" to make this work.
See: http://jsfiddle.net/2kRJv/
HTML:
CSS:
#Navigation
{
list-style-type: none;
text-align: justify;
height: 21px;
background: #ccc
}
#Navigation li
{
display: inline
}
#Navigation .stretch {
display: inline-block;
width: 100%;
/* if you need IE6/7 support */
*display: inline;
zoom: 1
}
Details on IE6/7 trickery: Inline block doesn't work in internet explorer 7, 6