Ref.: http://stackoverflow.com/questions/7647195/evenly-spaced-list-items
made a jsFiddle of your menu… everything is perfectly spaced, dynamic, and it goes all the way to the left/right edges without JavaScript or weird/ugly HTML semantic issues. (And it should work in IE 6, if it matters.)
http://jsfiddle.net/bXKFA/2/
HTML:
CSS:
#menuwrapper {
    height: auto;
    background: #000;
    text-align: justify;
    -ms-text-justify: distribute-all-lines;
    text-justify: distribute-all-lines;
}
.menuitem {
    width: auto;
    height: 40px;
    vertical-align: top;
    display: inline-block;
    *display: inline;
    zoom: 1
    background: #000;
    color: yellow;
}
.stretcher {
    width: 100%;
    display: inline-block;
    font-size: 0;
    line-height: 0;
}