$("#myModal").modal().css({
"margin-top": function () {
return - ($(this).height() / 2);
}
});

that.$element.children().eq(0).css("position", "absolute").css({
"margin":"0px",
"top": function () {
return (that.$element.height() - that.$element.children().eq(0).height()-40) / 2 + "px";
},
"left": function () {
return (that.$element.width() - that.$element.children().eq(0).width()) / 2 + "px";
}
});

<div>
<button class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
Launch demo modal
</button>
<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
<h4 class="modal-title" id="myModalLabel">Modal title</h4>
</div>
<div class="modal-body">
...
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
<button type="button" class="btn btn-primary">Save changes</button>
</div>
</div><!-- /.modal-content -->
</div><!-- /.modal-dialog -->
</div><!-- /.modal -->
</div>