OK, here you go, seems to work. I've got 2 new Opportunity fields WebLeadQuantity__c (Number) and WebLeadProduct__c (Text). I pass my product code from my web form, not the pbe id (if you do this you can shorten the trigger slightly - but using the Product code means it will work in live and sandbox/developer edition).

Here's the trigger, replace the WebLead fields with your own and it should work (you can do it simpler if you never ever bulk load Opportunities):

* Function: If we have any web leads with products and quantity then create a line item */
trigger create_WebOpportunityLines on Opportunity (after insert) {

    // Get a list of all the WebProduct codes for our opportunities
    List<String> prodList = new List<String>();
    for (Opportunity opp : trigger.new) {
    	if (opp.WebLeadProduct__c != null) prodList.add(opp.WebLeadProduct__c);
    }
    
    // Create a map of the applicable pricebookentrys so we can reference unit prices 
    // without breaking governor limits
    Id standardPb = [SELECT Id From PriceBook2 WHERE isStandard = true][0].Id;

    Map<String, PriceBookEntry> pbeMap = new Map<String, PriceBookEntry>();
    List<PriceBookEntry> pbeList = [SELECT Id, UnitPrice, ProductCode 
    				      FROM PriceBookEntry 
    				     WHERE ProductCode IN :prodList 
    				       AND PriceBook2Id = :standardPb]; 
    // Populate the map with the list items, keyed by product code
    for (PriceBookEntry pbe : pbeList) {
    	pbeMap.put(pbe.ProductCode, pbe);
    }
    
    List<OpportunityLineItem> oliList = new List<OpportunityLineItem>();
    for (Opportunity opp : trigger.new) {
        // Check if this is a web lead - ideally expand this with the lead source to identify
        if (opp.WebLeadQuantity__c != null && opp.WebLeadProduct__c != null) {
            OpportunityLineItem oli = new OpportunityLineItem();
            oli.OpportunityId = opp.Id;
            oli.PriceBookEntryId = pbeMap.get(opp.WebLeadProduct__c).Id;
            oli.Quantity = opp.WebLeadQuantity__c;
            oli.UnitPrice = pbeMap.get(opp.WebLeadProduct__c).UnitPrice;
            oliList.add(oli);
        }
    }
    
    if (oliList.size() > 0) insert oliList;
    
}