Prepopulating a multi-select on a webform
I have just read through the code of Prepopulate module, and it seems that as for now it doesn't support the feature you're after. The prepopulate module just assigns whatever is passed as a parameter to #value of the corresponding form element. To have multiple items selected you need to assign to #value something like this: array('Value 1', 'Value 2'); but there is no way you can pass this via parameters.

THe possible workaround is to hack the module code. Add the following code on the line #78 of the file - prepopulate.module

  if($form['#type'] == 'select' &amp;&amp; $form['#multiple'] == TRUE) {
    $requestslice = explode(',', $requestslice);
  }


just before this line:

  $form['#value'] = $requestslice;


This change will allow you to pass multiple values as a comma separated list, for example:

  mywebform?edit[submitted][multiselect_name]=Value1,Value2


I have just created a patch for for the prepopulate module that implements this thing, you can find it here:

http://drupal.org/node/1468024