As of this morning, Ubercart has a beta release for use with Drupal 6, and it's ready to have the snot kicked out of it by some hard-hitting testers. Most of the changes from Ubercart 1 are internal, so experiencing Drupal 6 will probably be the biggest difference. We have also trimmed down our dependencies so that Token is the only core requirement to use Ubercart. Image support still requires the Content module from CCK along with ImageAPI, ImageAPI GD2, Image Cache, Filefield, and Imagefield.
You can check out some of the improvements we've made and some known issues with the current code in the release notes at drupal.org.
We want to keep making Ubercart better, and that will be a lot easier once we know where the bugs are in this release. Once you've found one, head over to the issue queue and file a bug report (or voice your discovery in an existing issue). Some of them are sneaky, so make sure no one's beaten you to the punch.
At this point, do not update production sites running Ubercart 1.x on D5 to Ubercart 2.x on D6. We want folks with development environments to test the update process and report issues, but we do not recommend performing an update on a live site. If you want to test, you should make a copy of your database and file system and perform the update on the copy. Thanks.
Many thanks to our sponsors who helped various parts of core make it into Drupal 6: