[Adam H]

Just throw some css in the additional.css template like this :

.incontentnotice  {
border: 1px solid #0B1986;color:#000;background-color:#F5FFFF;padding:5px;
}

Then When you want to make a notice in the post wrap it in

<div class="incontentnotice">YOUR NOTICE HERE</div>

Obviously you have to have HTML switched on in your posts to be able to insert html into it.