On the heels of Microsoft's announcement that it will support OpenID in CardSpace, AOL has become the latest major player to support OpenID. "Every AOL/AIM user now has at least one OpenID URI". This is big news.
We made the decision to use OpenID as the sole authentication mechanism on a product we're building, and I'm increasingly happy with that decision. At best, we'll have no local accounts. At worst, we'll be an OpenID provider. It would be relatively trivial to expose ASP.NET membership as an OpenID provider (especially with the JanRain Server component). In fact, I'm somewhat surprised no one has done this already.
If you're not familiar with OpenID, check out the brief introduction to OpenID.