Important edit: Seems it doesn't work with Google Apps accounts - apologies to Zach in the comments, you were quite right. I'm revising this post. However, it does still work as described with all Google Mail domain accounts (e.g. personal Google accounts).

So, OpenID. Been about for a while. I kind of knew how it worked. I also knew I had dozens of OpenIDs, all in places of no use to me whatsoever that were generated automatically when I signed up for some service or other.