My feeling about single pane checkout from a customer point of view is that it is a lot of data to be getting right on a single screen. If a website is going to choke on my order, 95% of the time it's on the payment. The payment error may be caused by anything from my CC being temporarily blocked by the bank to a programming error to an unannounced ISP upgrade of PHP. Then either I get some ugly undecipherable error message on the screen, or get kicked back to the checkout page.
In the first case I lose all the data I entered. In the second case, after I get kicked back to the checkout page, often one or more of the fields that I filled in previously has blanked or mysteriously changed. So I have to go back and check everything, and I don't do that very well, because in my mind I had already checked the data I entered. After going through this a few times, I usually give up and go to Amazon.
If my address information and shopping cart were saved, I would be a lot likelier to complete the order once the payment problem was resolved. I think I saw an ubercart contrib for an ubercart address book. This might be really helpful.
One other thing that bugs me... I really don't like sites that say, after I press the "confirm order" button, only something like "Your order was received". I need to have notice if I'm about to miss the only chance of printing my order.

I really appreciate all the hard work that has gone into ubercart... it never ceases to amaze me what a good collaboration ccn accomplish!

Andrew