I implemented a registration procedure by creating a separate, non-authenticated page in Alfresco. I had a link on the Login form, something like "New here? Register for an account", pointing to that page. That page connected to admin-executed, non-authenticated backend webscript that created the user account, returned a login ticket etc. There are a few caveats though, so I'll write a longer post about it and let you know how I-ve done it here.
I've been thinking of doing that blog post for a while now, but have been postponing, and this week I have some extra time so I can finally get about it.
(Just in case I don't remember to post it here, check my blog, zlayer.net within the next few days.