I am quite uncaring for "how" the functionality is put in place as long as its robust and solid - does not even have to have much featuritits - just splitting each pane as it is now (as selected) into its own page would just do great.
As for "just rolling this into a D6 only" release - I am very very unhappy about that I have no plans to upgrade my site to D6 in the foreseeable future (it took me a year to have a D5 site running now and about 60% of the modules I use do not even have started a D6 port). So if you plan Ubercart 2 or whatever make sure its backward compatible with D5.

I do like the suggestion to save user input before the the transaction is in place A LOT! I too as a customer have had situations where I did not make the purchase because all my entered data was lost and there was no way to go back to where I was.

So a "this is your shopping cart at XXubercart store" email with login information and the current shopping cart and all information the user has put in would be a heaven send! (along with a customizable note like "if you do not recieve a order confirmation please click >here< to go to the payment page" or something along those lines...

Again thanks for the awesome work so far!