<script type="text/javascript">
function greyText(chboxname, textareaname) {
if (chboxname.checked) {
textareaname.style.color = "#000";
} else {
textareaname.style.color = "#999";
}
}
</script>

Called like this

<input name="showit" type="checkbox" value="" onclick="greyText(document.myformname.showit,document.myformname.mytext)" />

<script type="text/javascript">
function greyText(chboxname, textareaname) {
if (document.myformname[chboxname].checked) {
document.myformname[textareaname].style.color = "#000";
} else {
document.myformname[textareaname].style.color = "#999";
}
}
</script>

checkbox calls it like this:

<input name="showit" type="checkbox" value="" onclick="greyText('showit','mytext')" />

document.getElementById(textareaname).disabled = !(document.getElementById(chboxname).checked);

In other words, set the disabled attribute to the opposite of the checked value. If checked, disabled is false (ie, it works). If not checked, disabled is true (it works). The getElementById is just what I use for xhtml compatibility. You can use it if you add an "id" attribute to your fields. Otherwise your code will be fine - just change the method from .style to .disabled.

It probably isn't a big deal, most of the people using the page I'm working on right now, will almost certainly be on IE, but I was just wondering…

As is, it requires IDs on your fields. So as long as you have id="mytext" and id="showit" in the appropriate places, it should work fine. You can even do without the function by doing it like this:

<form name="formname">
<input id="showit" checked="checked" name="showit" type="checkbox" onclick="document.getElementById('mytext').disabled = !(this.checked);" /> Show this paragraph<br />
<textarea id="mytext" name="mytext" wrap="virtual" style="color:#000;">Lorem ipsum dolor sit amet no nummy blah blah blah….</textarea>
</form>

Note the id attribute on each field, and also the checked attribute on the checkbox. Otherwise it will start off un-checked, and you won't be able to see a difference when you check it.