The long delayed version 1.5.1 of WordPress MU has just been released. If you don’t want to read the rest of this post head to the download page and grab the zip file or tarball but make sure you come back here to read the upgrade docs.
This release of the popular multi-blog version of WordPress is synced with WordPress 2.5.1 and so has all the great features as well as bug and security fixes that went into that release.
Upgrading from a previous release
As long as you haven’t modified any core files, you can copy the files in 1.5.1 over your current install. Database upgrades will happen transparently in the background. The new salted hashing on passwords requires two constants, SECRET_KEY and SECRET_SALT to be defined in wp-config.php. If you upgrade and you don’t change wp-config.php your users will appear logged out when they go to a different blog. That’s why MU will display an ugly warning message to site admins with the two lines when they log in to the backend.