Currently it only works for the .dropdown-menu:
.dropdown-menu .divider {
height: 1px;
margin: 9px 0;
overflow: hidden;
background-color: #e5e5e5;
}
If you want it for other use, in your own css, following the bootstrap.css create another one:
.divider {
height: 1px;
width:100%;
display:block; /* for use on default inline elements like span */
margin: 9px 0;
overflow: hidden;
background-color: #e5e5e5;
}