The job of the CDS is quite simple. It runs on a single well-known domain, and providers a pointer to a user's PDS. The only user interface required is to get the user's consent to set/modify their PDS on a particular browser.
The following is a summary of the requirements for the CDS:
A single global domain is setup, such as cds.com
If a user is redirected to cds.com/setpds?dm=<domainname>&continue=<URL destination at domainname> then the cds.com webserver will ask the user "Please confirm you would like all the websites you visit to be able to determine that <Domainname> has your web browsing preferences."
If the user says yes, a cookie on cds.com is set with the value of that domainname, and the time at which it was set.
If the user already had a PDS set, the question above would be appended with "(Your previously had listed <olddomainname> as the location of your web browsing preferences)"
The site will then support cross-site preference lookup techniques by any website to learn the current domainname/time for that browser, and the cds.com server would echo back the values stored in the browser's cookie
This service will need to be free, but will get heavy load for the cross-site requests, so we may need a neutral group/company similar to Verisign to run it (though they may in turn outsource the operations to someone like Google/Verisign/Microsoft)
The statistics about the service will probably need to be made publicly available (such as usage levels and popularity of different personal discovery services)
The CDS only needs to store a single small preference (it fits in a cookie) so it will scale easily.
The CDS does not need a login system.
One partial alternative to the CDS is to define a standard URL parameter that can be used to pass a user's PDS. That way, if one site knows a user's PDS and is redirecting the user to another site, then it could pass that PDS information as a hint. In particular, a websearch or E-mail provider might learn of (or actually be) a user's PDS, and it could add that URL parameter whenever the user clicked a search result, and ad, or a link in E-mail.