title: KnockoutJS: based on length of array enable button
body: <p>In a project we're doing, we have created an inbox where the users (amongst other things) can select the items. If a user selects an item, the button should be enabled -- if none are selected, the button should be disabled. </p>
  
  <p>Simple enough, but for the life of me, I can't get it working :-( </p>
  
  <p>You can find a fiddle here: <a href="http://jsfiddle.net/rzrfp/" rel="nofollow">http://jsfiddle.net/rzrfp/</a></p>
  
  <p>I presume I'm missing something very, very, VERY simple and stupid, but I've been tried virtually everything, and can't get it working ...</p>