Donncha, doesnt stop working for the WPMU community. He just annouced the release WP-Hashcash plugin for WordPress Mu modified to work on the signup page and help stop splogs automated signups.
This is the description of the plugin and it features:
WP Hashcash is an antispam plugin that eradicates comment spam on WordPress blogs. It works because your visitors must use obfuscated javascript to submit a proof-of-work that indicates they opened your website in a web browser, not a robot. If the javascript check fails, WP Hashcash now gives you three options; it can either put the comment into moderation (default), put the comment in the akismet queue, or delete it.
Features
- Blocks all comment spam, but not real comments
- Also prevents most trackback / pingback spam
- Widget support to display spam statistics and edit the configuration
- Works with IE, Firefox, and Safari
- 100% standards compliant XHTML 1.1
- Tested with Wordpres 2.3, Firefox 2, Safari 3, and IE 7
- Akismet compatibility
To install WP Hashcash, just download the plugin and unzip it, then copy the wp-hashcash.php file to wp-content/mu-plugins and visit “Site Admin” -> “WordPress Hashcash” to confirm it’s working.
Download: WP-Hashcash for WPMU
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July 10, 2008 
thanks.. im just running it now on wpmu and buddy press and ill let you know how it goes..
Is this still kept up to date anywhere?