I have this observer listening: sales_quote_product_add_after
Inside the observer I run this code to change the price of the item when added to the cart.
$item = $items[0];
$request = Mage::app()->getRequest();
$newPrice = $request->getParam("new_price");
$item->setCustomPrice($newPrice);
$item->setOriginalCustomPrice($newPrice);
$item->getProduct()->setIsSuperMode(true);
$item->save();
This works perfectly when a customer is logged in, however when a customer isn't logged in I get a red error message on the front end saying: "Cannot add the item to shopping cart." and it doesn't add to the cart.