function enabled_disabled(){
		if(document.registerform.post.value==2){
			document.registerform.category.disabled=false;
		}
		else{
			document.registerform.category.disabled=true;
		}
	}

<form name="registerform" id="registerform" action="#" method="post">
<p><label for="id_post">Тип поста:</label>
			<select name="post" id="post" onChange="enabled_disabled();">
				<option value="0">Выбрать</option>
				<option value="1">Новость</option>
				<option value="2">Статья</option>
			</select>
		</p>
		<p><label for="id_category">Категория:</label>
			<select name="category" id="category" disabled="disabled">
				<option value="0">Выбрать</option>
					<option value="1">Python</option>
					<option value="2">GUI</option>				
			</select>
		</p>
</form>