The APEX Language Reference and Force.com Developers Guides (among others) state that the New collection can be written to in a Before Insert trigger.   Unfortunately when I try to do so, I get a 'Collection is read-only' error.   What gives?

 Sample code:

trigger ProductKits on OpportunityLineItem (before insert)
{

//test, just add an opportunitylineitem to trigger.new
            OpportunityLineItem bLineItem = new OpportunityLineItem();
                    bLineItem.quantity = 1;
                    bLineItem.UnitPrice = 100.00;
                    bLineItem.Opportunityid = '006S0000002pi1Y';
                    bLineItem.pricebookentryid = '01u80000003LOTJAA4';
            trigger.new.add(bLineItem);

}