Z-index no me funciona!

[taxiarcos]

Perfecto me funciona en Firefox, en Safari/chrome pero no en explorer 7 (ni os cuento en explorer 6) como se llega a ver!!!

Que le sucede a explorer???

Código:

*
{
  margin: 0;
  padding: 0;
  font-family: Arial, Verdana, Tahoma;
  font-size: 12px;
}

.clearer
{
  clear: both;
}

body
{
  background-color: #583D1F;
}

#container
{
  background: url('../images/bannerhome_01.jpg') no-repeat top left;
  width: 1000px;
  margin: 0 auto;
}

#header
{
  text-align: right;
  padding: 80px 80px 0px 0px;
  height: 30px;
  margin: 0 auto;
}

#header a
{
  border-bottom: 2px solid transparent;
  color: #362616;
  font-size: 14px;
  font-weight: bold;
  margin-left: 20px;
  padding-bottom: 1px;
  text-decoration: none;
  letter-spacing: -0.5px;
  text-transform: uppercase;
}

#header a:hover
{
  border-bottom-color: #C16609;
  color: #C16609;
}

#content
{
  margin: 0 auto;
  width: 807px;
}

#page_line
{
  background: url('../images/pageline.png') top left;
  width: 807px;
}

#page_up
{
  background: url('../images/pageup.png') no-repeat top left;
  width: 807px;
  height: 41px;
  z-index: 5;
  position: relative;
}

#page_down
{
  background: url('../images/pagedown.png') no-repeat top left;
  width: 807px;
  height: 60px;
}

#footer
{
  padding-left: 10px;
  padding-right: 10px;
  margin: 0 auto;
}

#footer .line
{
  float: left;
  color: #FFFFFF;
  font-size: 12px;
  padding-top: 2px;
}

#footer .line_right
{
  float: right;
  color: #FFFFFF;
  font-size: 12px;
  padding-top: 2px;
}

#footer .line_right a
{
  color: #FFFFFF;
  text-decoration: none;
}

#footer .line_right a:hover
{
  text-decoration: underline;
}

#footer .line_right img
{
  border: 0 solid transparent;
  position: relative;
  top: 3px;
  margin-left: 5px;
  margin-right: 5px;
}

#text
{
  margin-left: 45px;
  margin-right: 25 px;  
}

#sub_menu
{
  position: relative;
  text-align: right;
  margin: 0 auto;
  right: 40px;
  top: 40px;
  padding-bottom: 80px;
}

#sub_menu img
{
  border: 0 solid transparent;
}

#sub_menu .front 
{
  padding-left: 20px;
  float: right;
}

#sub_menu .back 
{
  padding-left: 20px;
  float: right;
}

#sub_menu .front img
{
  z-index: 10;
  position: relative;
}

#sub_menu .back img
{
  z-index: 1;
  position: relative;
}

.tit
{
  font-size: 15px;
  font-weight: bold;
  margin-bottom: 5px;
  margin-top: 7px;
}