any reason why the assignment rules won't trigger for new cases in this trigger:

trigger WebCaseAssign on Case (before insert) {
    integer i = 0;

    for (Case theCase:trigger.new) {
        if(theCase.Origin == 'Web') {
            Database.DMLOptions dmo = new Database.DMLOptions();
            dmo.assignmentRuleHeader.useDefaultRule = true;
            theCase.setOptions(dmo);   
        }
       
    i++;       
    }
   
}

I assume it's related to this issue/bug with Salesforce:

http://community.salesforce.com/sforce/board/message?board.id=Visualforce&message.id=10690