Release candidate 1 is out today: Grab it from http://mu.wordpress.org/wordpress-mu-2.6-rc1.zip
Changes in this release include:
1. Redirecting to the signup page for 404s and for unknown blogs is not enabled by default. Check out wp-config-sample.php for instructions.
2. "allowed_themes" filter, much like the plugins filtered added previously.
3. New functions: get_id_from_blogname(), is_main_blog().
4. get_blog_details() can now take a blogname as well as a blog_id.
5. Custom first posts didn't always work. Now they do.
6. Blognames in the "Add blog" form in wpmu-blogs.php are now sanitized.
7. Added "pre_site_option_*" and "site_option_*" filters like the similar option filters.
8. Meta fields will be passed on signup again.
9. Added an "admin_header_navigation" filter so the top right navigation in the backend can be customised.
10. The signup page uses "blogname" instead of "blog_id" to avoid confusion with the global variable of the same name. Plugins will break if not updated!
If you haven't read Ryan's post about cookies in WordPress 2.6 you should go there now. We had to take a few shortcuts in MU so users could still login on each blog which explains all the constants defined in wp-config.php.
One of the oddities of those changes is that visiting wp-login.php doesn't log you in again. Visiting wp-admin/ directly will work however. An easy way around this is wp_loginout() which prints a link to wp-admin if you're logged in, or wp-login.php if not.
Final WordPress MU 2.6 release very soon.