I'm running Wordpress Mu 1.3.3 on a custom server, mixing custom domains and subdomain blogs. I wanted a custom login that would look different for each blog, without a lot of hassle.
So, I hacked up Binary Moon's Custom Login for Wordpress:
http://www.binarymoon.co.uk/projects/bm-custom-login/
At first, I tried calling $current_site->id into the plugin, but that didn't work. So I took the 'echo' out of the function and got rid of the plugin all together.
So, I went into wp-login.php and right after the globals call on line 30 I put:
$bm_site_id = $current_site->id;
Then, right before the end of the <head> tags I put:
<?php echo '<link rel="stylesheet" type="text/css" href="' . get_settings('siteurl') . '/wp-content/plugins/bm-custom-login/' . $bm_site_id . '.css" />'; ?>
Then, in the plugin folder, I just named css files 1, 2 etc based on the site id.
I'm sure there's a cleaner way of doing it, but that's what I got.
You can view it at:
http://www.noreasoncomics.com/wp-login.php
http://thewatchtowerpodcast.com/wp-login.php