When trying to register for an account, the OpenID is not recognized, as shown in the following image:
However, the login being presented is not a new login, and is the same login used for his English Language & Usage Stack Exchange account. This has been confirmed to happen even with a clean cache and no potential confusion by the system that he is logging in with a different credential.
While this is consistent for the blog entry, Robusto had also noted that this occurs occasionally when trying to log into chat from a new device, or when logging into an account on a site he hasn't visited in a long time. This leads me to believe that there's something deeper at issue here than just the blog, but something else about his OpenID not being recognized.
If anyone else has been experiencing issues where your login credentials are not being recognized as existing on the Stack Exchange Network, when indeed they do, please speak up and provide any extra detail that you might have.