It is so easy to enable Google, Facebook Connect, Twitter, and Hotmail account logins for your site that it makes absolutely no sense to roll your own. Instead of rolling your own features like login, signup, forgot-my-password, confirm-email etc., just implement the above for your sites from the get go. OpenID is good too but I've found almost nobody uses that to log into my sites. 75% are Gmail/Google, 20% Facebook, and rest are Twitter and MSN.
<i>"If you want another company to use your services, then you need to make it easy."</i><p>Or, make a service that's so good, enough people <i>demand</i> integration.<p>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.