prova così (IE, Firefox)

codice:
<TEXTAREA id="ta1" style="WIDTH: 208px; HEIGHT: 136px" rows="8" cols="23" language="javascript" onkeyup="ta1_onkeyup(event)"></TEXTAREA>
codice:
function ta1_onkeyup(e) {
	var code;
	if (!e) var e = window.event;
	if (e.keyCode) code = e.keyCode; 
	else if (e.which) code = e.which;
	var character = String.fromCharCode(code);
	var target = (e.target)?e.target:e.srcElement;
	//alert(target.id + '\nCharacter ' + code + ' was ' + character);
	
	if(code == 13) 
	{//alert(target.id + '\nCharacter ' + code + ' was ' + character);
		if(window.event)
			target.value = target.value.replace(/\r\n/g, "
\r\n").replace(/

/g, "
");
		else target.value = target.value.replace(/\n/g, "
\n").replace(/

/g, "
");
	}
	
	
	
}