This foobar has some HTML in the message to show a newsletter signup form. The HTML added is

<form id="subscribeform" action="http://feedburner.google.com/fb/a/mailverify"
method="post" target="_blank">
  <span>
    <label for='email'>Sign Up To My Newsletter</label>
    <input id='email' class='subscribeInput' type='text' name='email' value=''>
  </span>
  <input type="hidden" value="themergency" name="uri"/>
  <input class='subscribeSubmit' type='submit' value='Go!'>
</form>

Here is the custom CSS to make it look pretty:

.subscribeInput
{ 
  border: 1px solid #E5E5E5;
  font-size: 15px;
  height:28px;
  margin-right:2px;
  outline: none;
  width: 150px;
  color:#000;
  -webkit-border-radius: 3px;
  -moz-border-radius: 3px;
  border-radius: 3px; 
}

.subscribeSubmit
{
  width:60px;
  height:28px;
  border:0px !important;
  background:#369 !important;
  display:inline;
  color:#fff !important;
  cursor:pointer;
  font-size:14px;
  position:relative;
  text-shadow:0 2px 2px rgba(0, 0, 0, .4);
  font-weight:bold;
  -webkit-border-radius: 3px;
  -moz-border-radius: 3px;
  border-radius: 3px; 
}

I have hooked the form up to my feedburner account, so it is an actual working example.