Accounts get in the way of startups working together
sachin.posterous.com
sachin.posterous.com
Sure, maybe some of those would have used RPX if that were the only thing we had, but apparently they still prefer a username.
Going 100% with the 3rd party logins is a tricky route. It's impossible to know how many potential signups you're losing.
What happens to your startup if (though unlikely) in six months time Twitter decides to start charging for authentication or different levels of service?
Something to consider.
If you'd like assistance with optimizing the user experience to maximize 3rd party login success rate, please contact johnb at janrain.com
Or, make a service that's so good, enough people demand integration.
This article makes some good points but I think things change once you start talking about startups that charge money ... and the Typekit example he gives is proof. It's the one paid service he lists–it doesn't use RPX, FB connect, etc.