Sub-blog on Network site redirects to wrong URL for user registration

[amereservant]

When a user clicks on the register button of a sub-blog, it redirects the user to the signup form on the primary blog, which means they end up creating an account with it instead of the sub-blog.

When the user tries accessing the sub-blog login after activation, they are denied access because the registration took place with the parent blog instead. There's also no way to transfer users to another blog.

This issue sounds similar to the one in #16866 , but it's not a feature request, instead it's a bug.

My test case is:
network blog is installed in sub-directory, such as ​http://mydomain.com/blogs/.
A sub-blog is created at say ​http://mydomain.com/blogs/my-sample-blog/.
I register a new user account with the sub-blog, which takes me to ​http://mydomain.com/blogs/wp-signup.php instead of ​http://mydomain.com/blogs/my-sample-blog/wp-signup.php.

Running on Ubuntu 10.04, Apache 2, PHP 5.3, WordPress 3.2.1.

I've found the problem ... in wp-signup.php, line 30 redirects the user if !is_main_site(), so it takes them to the signup form on the main site instead, thus causing the user to register with the wrong site.

Related: #17948

This is not a bug. WordPress MU worked exactly the same way.