Index: wp-signup.php
===================================================================
--- wp-signup.php	(revision 22227)
+++ wp-signup.php	(working copy)
@@ -30,6 +30,14 @@
 // Fix for page title
 $wp_query->is_404 = false;
 
+/**
+ * Prints styles for front-end Multisite signup pages.
+ *
+ * The default styles are printed via signup_header which is hooked to wp_head. 
+ * They can be overloaded by replacing the output on the signup_header action hook.
+ *
+ * @since 3.0.0
+ */
 function wpmu_signup_stylesheet() {
 	?>
 	<style type="text/css">
@@ -50,7 +58,6 @@
 	<?php
 }
 
-add_action( 'wp_head', 'wpmu_signup_stylesheet' );
 get_header();
 
 do_action( 'before_signup_form' );
Index: wp-includes/default-filters.php
===================================================================
--- wp-includes/default-filters.php	(revision 22227)
+++ wp-includes/default-filters.php	(working copy)
@@ -213,6 +213,7 @@
 add_action( 'wp_print_footer_scripts', '_wp_footer_scripts'                 );
 add_action( 'init',                'check_theme_switched',            99    );
 add_action( 'after_switch_theme',  '_wp_sidebars_changed'                   );
+add_action( 'signup_header',       'wpmu_signup_stylesheet'                 );
 
 if ( isset( $_GET['replytocom'] ) )
     add_action( 'wp_head', 'wp_no_robots' );