Sign in WPmu with usename having mixed characters (lowercase+uppercase) is OK, but user can not log in his/her blog with the password sent out to his/her email (password error).

We can fix this issue by accessing to database and changing username to all-lowercase.

My method:

Change line 307 in wp-signup.php:

$user_name = $filtered_results[’user_name’];
to
$user_name = strtolower($filtered_results[’user_name’]);

will quickly fix this issue.

Now the username is converted to all lowercase.

See detail of my suggestion on WordPressmu forum.