It's my understanding that Ubercart 3 will just be a direct port of 2 to Drupal 7.
There's also a fork of Ubercart for Drupal 7, called Drupal Commerce. This will be a complete re-write of Ubercart, taking what the developers have learned over the years working on Ubercart.
You can read more about the Drupal Commerce project here:
I would expect most of the heavy lifting related to Drupal 7 and E-Commerce to get done there and not in the Ubercart 3.x project.
Someone correct me if I'm wrong...but that's my understanding of the situation and one I support.