Reduce the ambiguity in creating blog and users and some additional checking

Donncha: When creating blog using the Site Admin -> Blog.

Do you want to add an additional input box for username? This will reduce the ambiguity in error echo by the following code and it's easier for new siteadmin to understand.

WPMU use blog domain as his/her username.

$user_id = email_exists($email);
if( !$user_id ) { // I'm not sure what this check should be.
	$password = generate_random_password();
	$user_id = wpmu_create_user( $domain, $password, $email );
	if(false == $user_id) {
		wp_die( __("<p>There was an error creating the user</p>") );
	} else {
		wp_new_user_notification($user_id, $password);
	}
}

Thanks ktlee, applied in [1122]

I think it's better to keep the form as simple as possible and once the email is received by the new blog owner it'll be obvious what they login as. It might be good to spell this out more clearly perhaps.