When it comes to identity, a Service Provider (SP) or Relying Party (RP) has a couple of key issues. One is customer facing (user experience) and another is technical (supporting multiple identity protocols). Of these two, the user experience is more important as it affects the consumer.
Take the following use case. My mother-in-law goes to a cooking web site to search for a new coffee-cake recipe. The cooking site (SP) allows for personalization and saving of favorite recipes, so she is presented with UI to login so she can save the recipe as a favorite (we'll assume that the “create the account” process has already occurred). The cooking site wants to support the widest range of people possible to access their site so they decide to support Cardspace, OpenID, and SAML at the identity protocol layer. Questions? How does the SP represent the UI so that it's not confusing to my mother-in-law? She doesn't understand anything about Cardspace, OpenID or SAML. She does understand that she has an AOL account.
Does this mean SP/RP's should be putting up icons of well known identity providers? That solution is not equalitarian and it doesn't scale. It's great that we have all these cool technologies that tackle different parts of the identity space, but it doesn't really help consumers if it is too confusing to use. I suspect most users know who they have an account with but not many know what protocol is used by their account. For example, I suspect that most Live Journal and AOL users do not know that they have an OpenID.
As for the technology side, the problem at the SP/RP is not that the issues are unsolvable (or even not currently solved). The problem is that all this code has to be written (or incorporated) and glued together to enable these different identity protocols. That is a rather significant barrier to entry. Currently, this tends to drive SPs to pick one of the existing identity protocols. From a business perspective this limits the total number of people that can use the service.
I tend to agree with Johannes, all this talk and work won't mean much if we don't make things simpler for the consumer first and the SP developer second.