Could be a good idea. One of the things I would like to have is a more general registration system, that was used for the forum, and could be used for other sites with more custom functionality.
Perhaps having a class such as RegistrationForm, that could be placed on a page and would let people fill out their details and become a member of the site - site implementors would choose:
* the data fields that were collected
* the permissions that people have once they were given access
* the restrictions that needed to be met to get that access (nothing, email validation, administrator approval).
On a particular, site, we could specify that such a registration form was available, and this could then suggest that failed log-ins (both standard and openid) go to the register page to sign up for an account.
Markus - does this sound like a good way to approach this problem?