Owkay I spent a few hours to figure out how to make the tags working.
the situation was the following:
I had a previous version of wp installed, and Ultimate Tag Warrior plugin
After installing the WP 2.3 I realized that I need my tags but couldn't figure out from where to take them.
If you are in this situation do the following:
after you installed WP 2.3 don't reactivate UTW it won't work, will give you some errors. Instead go to (main manu) Manage->Import scroll down a bit and you will see:
Ultimate Tag Warrior | Import Ultimate Tag Warrior tags into the new native tagging structure.
an easy to use importer.
now hopefully you won't lose a few hours to research the codex to find it
Also I use
http://www.herewithme.fr/wordpress-plugins/simple-tags Simple tags to manage the new tag system
good luck