When a customer makes an error on the checkout form such as, missing city in delivery panel, other data, notably the shipping quote and CC info are lost and the form repaints.

It appears that modules not defined in the initial uc_cart_checkout_panes do not get their data preserved in the forms api but am not positive of this.

The reason that this is critical is that a customer will screw up one part of his form, go back and fix it but other data will be missing. He will fix the screw up but the form will now fail on the additional missing data. Some set of customers get frustrated and abandon their carts. We can't measure this segment because they never make it to in-checkout.