There are several different web services -- various technologies used, such as Java, .NET, Python, Perl, and possibly more in the future -- belonging to different organizations,
We did a big research on the subject and couldn't find a suitable solution too. (One nearly good solution, but not so much for webservices is http://www.atlassian.com/software/crowd/)
So we developed a sso and central user management system too for our WS applications (also third party apps) but it's not for sale.
If you test solutions, you should check the performance of the systems, special under load. In the beginning our systems were 30 times slower. Normally you'll find the slow down in the xml parsing and the number of request you have to do (normally where you had one request in the future you'll have at least 4). (We use jmeter to test it.) And you should setup fail over systems, because you'll create a single point failure with sso.