I've actually implemented multipage checkout, because it seems to be the way our designers prefer to do things (I'm just a code monkey

Basically as Ryan said it's just about building the order object as you go along. I implemented this as I was learning ubercart, so there are alot of things I would like to change.

My current solution is more of a complete rewrite of the "GUI" of ubercart, using small adaptor modules to adapt things like shipping to my checkout process, and basically using Ubercart as an API and backend solution.
One of the main things I would like to do would be to take ubercarts "panes" and maybe abstract them so that they could be "pages".
I think it would be cool to be able to string a series of panes together through an admin interface, at some point I'd like to go back over my code and investigate the possibility of doing this.

If you guys are going to set up a project to do this, please PM me, I'm keen to help but only check the ubercart forums every few weeks.