Where should the authentication function go?

[eluser]Colin Williams[/eluser]
To the original topic, I've actually just considered rewriting a User library I wrote as a Model. I don't know that I like the Model interacting with the session and doing the access checks, among other things. But, I wouldn't want to have it all scattered across a user model, user library and user helper (which might be appropriate). I don't know. I'm still up in the air about it :-P