There are two ways to allow people to register directly to sub-blogs of Wordpress MU:
- using the mu-plugin written by Nathan Kinkade;
- changing
wp-login-php(than calling it aswp-login.php?action=register).
The first way is cleaner, because it doesn’t hack the core: therefore it doesn’t lead you to have a patch to maintain but I wasn’t able to let it working with the last stable 1.5.1 MU version (configured for sub-blogs showed as sub-directories: I’ve just written an email to Nathan so I’m waiting his notes).
The second choice is simple, it has the previous disadvantages but it worked (you can download here my trivial patch).
Update (2008-07-01): the new version of perBlogRegistrations works well just with a plain, unpatched WordPress MU (try to signup calling wp-signup.php).