This mimics the HTML5 placeholder in browsers that don't support it. Requires jQuery - can be changed for earlier versions by converting the .on() event calls. The "blur" class was used to apply a lighter gray color to the text.

placeholder.js

var placeholderSupport = ("placeholder" in document.createElement("input"));

var setPlaceHolderText = function(elem) {
  var $textarea = $(elem);
	var placeholderText = $textarea.attr("placeholder");
	$textarea.data("placeholder", placeholderText);
	if (placeholderText.length > 0) {
		$textarea.val(placeholderText)
					.addClass('blur');
	}
};

var getKey = function(e) {
	if (e.keyCode) {
		return e.keyCode;
	} else if (e.which) {
		return e.which;
	}
};

var isAlphanumeric = function(e) {
	var key = getKey(e);

	return (/^[a-z0-9 ]/gi).test(String.fromCharCode(key));
};

var positionCaret = function(elem, pos) {
	var range = elem.createTextRange();
		range.collapse(true);
		range.moveEnd('character', pos);
		range.moveStart('character', pos);
		range.select();
};

// onload comment areas
$('textarea').each(function(i, el) {
	if (!placeholderSupport) {
		setPlaceHolderText(this);
	}
});

// textarea events
$('textarea').on('click focus input change keydown keyup', function(e) {
	var $textarea = $(this);
	if (!placeholderSupport) {
		var placeholderText = base.$commentarea.data("placeholder");

		if (this.value === placeholderText) {
			positionCaret(this, 0);
		} else if (this.value === "") {
			this.value = placeholderText;
			$textarea.addClass('blur');
		}

		if (isAlphanumeric(e)) {
			if (this.value === placeholderText) {
				this.value = "";
			}

			$textarea.removeClass('blur');
		}

	}

	// handle IE ignoring maxlength
	var str = $(this).val();
	var mx  = parseInt($(this).attr('maxlength'), 10);
	if (str.length > mx) {
		$(this).val(str.substr(0, mx));

		return false;
	}
}).on('blur', function(e) {
	var $textarea = $(this);
	if (!placeholderSupport) {
		$textarea.val($textarea.data("placeholder"));
		$textarea.addClass('blur');
	}

});