They say good things come to those who wait, and there are many good things in this version of Ubercart. Product images now support Colorbox, which functions like Thickbox and Lightbox2, but also has a version for Drupal 7. The definitions of cart panes and checkout panes can now be altered by other module.
The biggest change, though, is that order invoice templates now use the theme system to allow customizations. Instead of altering the module files directly, it is now correct to override them in the theme, just like node and page templates. This does mean, however, that your customized .itpl.php files need to change their tokens into theme variables. See the upgrade documentation page for details. *note: If you have previously customized your invoice templates, you must update them to the new format (.tpl.php).
Thanks in large part go to TR, who has helped maintain the issue queue and fix many, many bugs.
Ubercart 3.0 Alpha 3:
No, your senses do not deceive you. Ubercart is now integrated with the Rules module! Rules has long been the standard way for Drupal sites to take actions as various events occur. Rules 2 for Drupal 7 will bring even greater flexibility and power to site administrators than ever before. And all of Ubercart's data will be available through the Rules interface and API. Ubercart is being developed with the latest versions of Drupal and Rules, so if anything breaks during testing, it shouldn't be long before a fix is available.
The short and sweet is, please test the 3.0 alpha 3 release. Alpha 3 IS NOT READY FOR PRODUCTION, but all the major changes for 3.0 are complete and it is ready for anyone and everyone "kick the tires." The upgrade path from 2.0 to 3.0 still needs some work, but most of the issues are dependent on completion of the Drupal 6- Drupal 7 upgrade path. As always, bugs, feature request, etc... can be posted to the issue queue.
Development Road Map:
With the release of 3.0 Alpha 3 we have achieved our primary goals for the 3.0. The goals were to port Ubercart to Drupal 7, to integrate with Rules, and to provide a set of "coder rules" to allow module developers to easily update Ubercart sub-modules to 3.0. We have a few small tasks yet to complete like menu cleanup, replacing the catalog with a default view etc... But at this point we are basically waiting on Drupal 7 to reach beta and the be released. We will continue to implement these small features until Drupal 7 reaches Beta (rumored to be in 2-4 weeks), and then proceed in lock step with Drupal through beta to a stable release. More information on the road map can be found on the official road map page and you can always make suggestions as comments on the road map, or as "feature requests" in the issue queue.
The Ubercart Team