I don't feel like the Bazaar version has been a moving target lately. Ever since we had an RC version, we've had a kind of unofficial code freeze. Or at least a code slushie. Very few features have been added while all the rest of the changes have been bug fixes and typo corrections. We will have an RC5 because Ryan wants to change the default credit card settings, and that needs more testing than fixing a typo. After that, if nothing tragic happens, we should release Ubercart 5.x-1.0.

Once that happens I'll open up the 6.x-1.0 branch. It'll start out as a copy of the D5 code and the porting work will happen there. New features won't be added in until the 6.x-1.0 version is ready and posted to drupal.org. During that time, only gross bugs that aren't taken care of by the D6 port will transfered between the two branches.

As for contributed modules, it's their maintainer's responsibility to keep up with the code. At the very least, they should say what version they are compatible with, starting with 5.x-1.0. That version of Ubercart will be available at drupal.org at least until Drupal 7 rolls out, and probably longer than that.