Phil almost has it - the only way to look at Typekey is as a centralized authentication mechanism. MT will implement in such a way that it facilitates comment moderation. Typepad will probably do the same. But this doesn't change what Typekey is at its core - a centralized authentication system that developers can embed in their own applications for their own purposes.

TypeKey from a different angle

I've been poking at the idea of TypeKey, and not much liking it, from the comment registration angle. I saw it as an answer to people begging for comment registration as though that would somehow make trolls go away, or stop spam or crapfloods, which it certainly won't: trolls and spammers and crapflooders are much more motivated, and thus much more willing to register with a throwaway address, than the sort of casual commenter who knows the answer your post was pleading for, and wouldn't mind giving it to you if it's not too much trouble.

Now, I'm not so sure that's the right angle to take. If instead of seeing TypeKey as a slightly more tolerable (because you only have to lie about your email address once instead of many times) way to implement registration, you look at it as way to implement comment moderation, it begins to look a tad bit better.