Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 19631)
+++ wp-includes/ms-functions.php	(working copy)
@@ -439,7 +439,7 @@
 	$maybe = array();
 	preg_match( '/[a-z0-9]+/', $user_name, $maybe );
 
-	if ( $user_name != $orig_username || $user_name != $maybe[0] ) {
+	if ( $user_name != $orig_username || ( !empty( $maybe ) && $user_name != $maybe[0] ) ) {
 		$errors->add( 'user_name', __( 'Only lowercase letters (a-z) and numbers are allowed.' ) );
 		$user_name = $orig_username;
 	}
Index: wp-signup.php
===================================================================
--- wp-signup.php	(revision 19631)
+++ wp-signup.php	(working copy)
@@ -229,13 +229,8 @@
 
 	if ( !is_wp_error($errors) )
 		$errors = new WP_Error();
-	if ( isset( $_POST[ 'signup_for' ] ) )
-		$signup[ esc_html( $_POST[ 'signup_for' ] ) ] = 'checked="checked"';
-	else
-		$signup[ 'blog' ] = 'checked="checked"';
 
-	//TODO - This doesn't seem to do anything do we really need it?
-	$signup['user'] = isset( $signup['user'] ) ? $signup['user'] : '';
+	$signup_for = isset( $_POST[ 'signup_for' ] ) ? esc_html( $_POST[ 'signup_for' ] ) : 'blog';
 
 	// allow definition of default variables
 	$filtered_results = apply_filters('signup_user_init', array('user_name' => $user_name, 'user_email' => $user_email, 'errors' => $errors ));
@@ -257,10 +252,10 @@
 		<?php } elseif ( $active_signup == 'user' ) { ?>
 			<input id="signupblog" type="hidden" name="signup_for" value="user" />
 		<?php } else { ?>
-			<input id="signupblog" type="radio" name="signup_for" value="blog" <?php echo $signup['blog'] ?> />
+			<input id="signupblog" type="radio" name="signup_for" value="blog" <?php checked( $signup_for, 'blog' ); ?> />
 			<label class="checkbox" for="signupblog"><?php _e('Gimme a site!') ?></label>
 			<br />
-			<input id="signupuser" type="radio" name="signup_for" value="user" <?php echo $signup['user'] ?> />
+			<input id="signupuser" type="radio" name="signup_for" value="user" <?php checked( $signup_for, 'user' ); ?> />
 			<label class="checkbox" for="signupuser"><?php _e('Just a username, please.') ?></label>
 		<?php } ?>
 		</p>