as I'm fresh here my vote is small - but I would give it for the following scenario:
- implement some low hanging fruits into an Ubercart 2.1 for Drupal 6
- save all the rest for Ubercart 3.0, Drupal 7

why?
- some features reach the community earlier when released in a v2.1
- probably can avoid some redundant work having v3.0 only for D7, thus it would be ready for D7 earlier