I might very well have a distorted vision (dream?) about the future of digital identity systems, but if we agree that they are going to be distributed and not centralized, this could be a non issue. Of course making identities distributed will not mean that a user will be able to choose his or her identity services from more than one provider, but that the very identity will not be some file saved on a specific server somewhere but will be a cloud of data across many different servers.
Some of these servers will provide services for persona use, some others for government or corporate applications, but they will still be facets of the same identity.
In the very good article there is a reference to a sentence of World of Ends:
. . . A new identity infrastructure?one provided by open APIs, protocols and other standards that serve no agenda other than to enable useful dealings between buyers and sellers of products and services. Like the Web and e-mail infrastructure that are already part of the Net, this new infrastructure would be a full-fledged service on the Net. And it won't become that unless it's something nobody owns, everybody can use and anybody can improve. Again, like the Web, e-mail and the Net itself.
The keywords in my opinion are like the Web and e-mail infrastructure. Even today many users are using more than one e-mail account: one for personal messages, one for professional communications and maybe even one to maintain anonimity on the web, but still, all these e-mail accounts are part of the system and can connect to each other.
With all this I mean that I believe that the real challange is going to be trying to develop a system where the same application will be applied to different environments.