I am in full agreement that the attributes of a product should be editable on the cart page. And on the checkout page, there should be a message to the effect of "to change attributes go back to the cart page". Viewing the cart or checking out is when your customer sees everything down in black and white and looks for errors - you want to keep your customer on these pages - forcing them to delete from cart then go back and re-enter a product with different attributes is a sure way to lose some customers who will never end up completing the checkout. You really want them to pay as soon as possible, without a chance for second thoughts, and the best way to do that is to give them what they need on the view cart and checkout pages.