Adaptive Path have put together a nice free whitepaper on Sign-up & Ramp up design patterns. I think it’s by Alexa Andrzejewski, one of their interaction designers.
One notable thing missing is the Lazy Registration design pattern, but hey, that’s a big enough topic for a whitepaper all of its own.
There’s a copy of the paper below, courtesy of Scribd, but it’s intended to be only available for newsletter subscribers. If you like it then go sign up.