WordPress MU

Why is "Only lowercase letters and numbers allowed" for user names only in WordpressMU but not in Wordpress?

My organization is used to usernames like:
<firstname>.<lastname>

and this is not possible when we move to WordpressMU.

What is the motivation behind this?

Thanks,

Clara Ko

I've seen this as well but if you add a user from the wpmu site admin it allows <firstname>.<lastname>. Haven't figured out if it can be changed but am interested because I know future blog admins will want to add users with . in the name.

You can use this plugin to change what username restrictions are:
http://wordpress.org/extend/plugins/wpmu-blog-name-restrictions-override/

Which file contains the text, "Only lowercase letters and numbers are allowed."?

I would like to edit it to say something like:

"Your user name cannot contain any symbols, punctuation, dashes or underscores and must be lowercase letters."

Many ... MANY users only see LOWERCASE and believe that "user-name" is fine, or "user_name" is fine since it is lowercase.

"I typed it in all lowercase, what's wrong??"

It's in wp-signup.php.