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.

If you run into trouble, remember to check the forum and Trac. Someone else may have already answered your question.
Thank you for all your hard work on this Donncha! Much appreciated.
After a fresh install I am redirected to /wp-admin/install.php, which is missing… What can I do?
Thank you
Thank you for great mu-1.5.1. I have to wait for it for a very long time 😉
Thanks tons, donncha! 🙂
Thanks for your great contribution to the wpmu community. I want to upgrade my 1.3.3 to 1.5.1 but i cannot find the install file and recommendation / issues with upgrade — could you point me in the right direction?
I have exactly the same problem than Murphy :/