Hola, prueba con esta función:
Código HTML:
<html>
<head>
<script>
function nl2br(str){
str=str.replace(/\n/gi,"<br />");
return str;
}
function reformat(){
/*....*/
document.dummy.area.value=nl2br(document.dummy.area.value);
/*....*/
}
</script>
</head>
<body>
<form name="dummy" id="dummy">
<textarea name="area" id="area" rows="10" cols="50"></textarea><br />
<input type="button" name="nl2br" id="nl2br" onclick="reformat()" value="nl2br" />
</form>
</body>
</html> Saludos