As someone's who's had to deal with the 'single sign-on' concept before, I can mention a few of the bigger items that I've run into.
- Change -- each site has a current system that works, and takes negligible effort to maintain. You are asking each site to modify their authentication system, which has the possibility of breaking their site.
- Scalabilitiy issues -- You can either set up one large centralized authentication system (LDAP, Kerberos, whatever), or set up some way for each site to authenticate off of the other sites. So it's either a single point of failure, or n(n-1) complexity (and becomes more complex as new sites are added).
- Incompatable user names -- my PAUSE id is 'ONEIROS'. My perl.org id is 'oneiros', and my id on here is 'jhourcle'. You would have to add complexity to deal with the fact that names may not map from system to system, and that the same id may be used by two different people across the systems. Trying to unify the systems after the fact is a massive PITA -- you can have each person create a new unified account, and then map all of their existing accounts back to it, but you get bickering when someone takes a userid that's been long associated with someone else (even if only one one system).
- Account management -- You need a way for sites to request password changes, or information changes, or to recover lost passwords, or whatever might be needed. Which leads us to...
- Trust -- and this is the biggest one -- certified logins only work when everyone trusts each other -- no one's going to authenticate as the wrong person, or otherwise claim to be someone else. They're not going to cache a password so that they can use it to gain access they might not have otherwise had. (AdminA on SiteA logs into SiteB. AdminB steals AdminA's credentials, so they can get admin access on SiteA; Or HackerC gets into SiteB, and then uses that to gain abnormal privs on SiteA and the rest of the network)
I'm not going to pretend that there aren't advantages to the users for single sign-on, but it is a royal pain to administer, especially if you're attempting to retrofit it into systems that already have active accounts.
Yes, it's possible, but there's a good chance that the risks and headaches that it would entail aren't worth it to the various site admins -- especially as it might be something that they aren't familiar with, and so it'd be a complete unknown risk.
If this is going to happen, I would assume it would start small, either centralized around one person who can unify the various sites, who is well trusted, and would take responsibility for the whole thing; or two sites who trust each other would band together, and possibly later add other sites into their circle of trust. I definately don't think this is as easy as someone saying 'make it so', and it happening overnight.