One problem all new web applications have is managing identities. Users need the ability to sign up and they need somewhere to login. And with all the other sites competing for their attention, you can bet they’ll need a way to recover their password (and probably their user id) when they forget it.
For most applications this is just plumbing that distracts from creating the good bits. No to mention that requiring a user to sign up is a hurdle to adoption. So why not make it so transparent the user doesn’t even realized they’ve signed up?
Joel’s article is about TripIt.com, which uses email as a sign up mechanism (although they do have a regular sign up page too).
