A new article is up online detailing how to build the basis of an adaptable distributed membership system.
It came about after struggling with migrating a userbase from an established site to a new portal. Giving users who have registered with you elsewhere the ability to sign in to your new site means you can both centralise your data and provide easy access to your existing membership.
Using web services, you can also build software applications that use your membership system, again, providing more opportunities.
Give it a whirl, and if you can see some enhancements, share them with everyone else by dropping a comment. Will be good to see what people do with the code! :)