a cheap and nasty would be:
jQuery("#myDiv").html().replace("
", "\n").replace("
", "\n")
EDIT
jQuery("#myTextArea").val(
jQuery("#myDiv").html()
.replace(/\
/g, "\n")
.replace(/\
/g, "\n")
);
Also created a jsfiddle if needed: http://jsfiddle.net/2D3xx/