Well, we were hoping to move from beta 5 to release candidates, but it's just not going to happen. We're more committed to a solid core than we are to hopping on through development phases as anyone who roughed it through the alpha phase can attest. There were too many minor bugfixes, particularly with a couple queries and admin pages, to consider moving on into release candidate territory. We'll let the beta 6 sit for a while to see what else pops up and maybe get a release candidate out before or during the Boston Drupalcon.
There aren't any major changes to report that would prevent you from updating. Mostly this is a release to propagate the query and uc_product_node_info() fixes that keep popping up in the forums. There have been a few other minor fixes and tweaks added to the code as well.
The latest release is available on our project page - http://drupal.org/project/ubercart
Refer to this FAQ page on updating your site if you're unsure how to do it or need a simple updating checklist.
You can check out our Bazaar log for commits since 2-8-08 for changes since the beta 5 release.
If any issues pop up, don't keep them to yourself. Let us know so we can get out of beta territory once and for all.
(Well, at least until 2.0!)
EDIT: There is a non-critical error popping up when you run update.php for an incorrect database query. It breaks nothing, will cause no problems in the future, and has been reported and fixed.

Go Ubercart!
I think some work has been done, but they're not issues the core team is able to work on atm.
