Stack Overflow converted my answer to a comment so I will post a more complete answer.
<div class="container"><div class="row-fluid"><div class="collapse-group"><div class="span2"><!--Sidebar content--></div><a class="btn" data-toggle="collapse" data-target=".span2">Toggle</a></div><div class="span10"><!--Body content--></div></div></div>span2 will now be collapsed by default and if you click the button it will expand vertically.