we definitely intend to keep up the 5.x version, but for our own personal use we need to move on in development. There are so many API changes that affect Ubercart that I anticipate at least a month converting the code over, and I'm loathe to start the two big remaining projects we have (eBay/QuickBooks integration) on Drupal 5 if I'm going to have to rewrite it all for Drupal 6.