I've examined the page you posted and I confirmed that the solution:
$('selector').val('value').slider('refresh');
does indeed work. Make sure that 'selector' is referencing your select element, and that 'value' is a value you defined on the option element you wish to enable.
I confirmed this by visiting http://jquerymobile.com/demos/1.0.1/docs/forms/switch/index.html, then with firebug's console entering the line:
$("#flip-b").val('no').slider('refresh');
It switched the second slider displayed on the page from yes to no.
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