Add this:
<script type="text/javascript">
function ClearForm()
{
main_form = document.getElementById('teh_form');
if ( main_form.value == 'Enter log details here...' )
{
    main_form.value = '';
}
}
</script>

Change This:
<textarea name="entry" rows="20" cols="90">
To:
<textarea name="entry" rows="20" cols="90" id="teh_form" onClick="ClearForm()">