Just get rid of the background color, borders and add hover effects. Here's a fiddle: http://jsfiddle.net/yPU29/
<form action="..." method="post">
<div class="row-fluid">
<!-- Navigation for the form -->
<div class="span3">
<ul class="nav nav-tabs nav-stacked">
<li><button type="submit" name="op" value="Link 1" class="button-link">Link 1</button></li>
<li><button type="submit" name="op" value="Link 2" class="button-link">Link 2</button></li>
<!-- ... -->
</ul>
</div>
<!-- The actual form -->
<div class="span9">
<!-- ... -->
</div>
</div>
</form>
CSS:
.button-link {
background-color: transparent;
border: none;
}
.button-link:hover {
color: blue;
text-decoration: underline;
}