KnockoutJS: based on length of array enable button
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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. 

Simple enough, but for the life of me, I can't get it working :-( 

You can find a fiddle here: http://jsfiddle.net/rzrfp/

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 ...