One more way – just add a selected = "selected" attribute to the select markup and call select2 on it. It must take your selected value. No need for extra JavaScript. Like this :
Markup
<select class="select2">
<option id="foo">Some Text</option>
<option id="bar" selected="selected">Other Text</option>
</select>
JavaScript
$('select').select2(); //oh yes just this!
See fiddle : http://jsfiddle.net/6hZFU/
Edit: (Thanks, Jay Haase!)
If this doesn’t work, try setting the val property of select2 to null, to clear the value, like this:
$('select').select2("val", null); //a lil' bit more :)
After this, it is simple enough to set val to "Whatever You Want".