There are theoretical optimums, like OpenID. Then there's what actually happens - popular sites spreading their ID system outwards. Facebook has just announced the latter:

On Monday, Facebook announced it will soon roll out its Facebook Connect login system to several high-profile websites, including Digg, Hulu and Discovery.com. The sites, along with a few others, will begin supporting Facebook Connect within a few weeks.

The difference between this and OpenID is simple; it will get used in the real world, while OpenID lives mostly in the dreams of Enterprisey types. Will there be convergence at some point? That's completely unclear to me. The thing to really watch for is whether Google and Facebook ever agree to a common way of doing identity. If that happens, you'll have your standard, regardless of what happens with OpenID.

On a related note, Dare Obasanjo did a great writeup awhile back on the challenges faced by people trying to use OpenID in the real world.

Technorati Tags: openID, authorization