The code you provided will enable the button correctly from my testing, however, the button having a disabled state does not disable the click event you've added. If you want the button to be truly disabled you need to bind the event only when you enable the button and unbind the click when you disable the button.
example can be found here: http://jsfiddle.net/WmAQJ/1/
<html>
<head>
<script type='text/javascript' src='http://code.jquery.com/jquery-1.5.js'></script>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.9/jquery-ui.js"></script>
<script type='text/javascript'>
$('#bewerkopslaan').button({
icons: {
primary: 'ui-icon-disk'
},
disabled: true
});
$('#enabler').click(function() {
$('#bewerkopslaan').button("option", "disabled", false).bind("click", function() {
alert('test');
})
});
$('#disabler').click(function() {
$('#bewerkopslaan').button("option", "disabled", true).unbind("click")
});
</script>
<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.10/themes/base/jquery-ui.css" type="text/css" media="all" />
<link rel="stylesheet" href="http://static.jquery.com/ui/css/demo-docs-theme/ui.theme.css" type="text/css" media="all" />
</head>
<body>
<div id="enabler">click me to enable button</div>
<span id="bewerkopslaan">button</span>
<div id="disabler">click me to disable button</div>
</body>
</html>