Claims based security is everything! It really is!
Especially in any SAAS, Heterogeneous or WCF-Services environment using Security Token Services is the technology to use... But honestly, it should be the security route for any solution!
I recommend reading this excellent document by David Chappel which gives a perfect conceptual (no code) explanation of what "Claims Based Access" and "Geneva" are about.
I also recommend reading this excellent document by Keith Brown which gives a perfect introduction (with code) to the whole "Geneva" suit!
For more informatie see msdn: http://msdn.microsoft.com/en-us/security/aa570351.aspx
Note: the "Geneva" framework cannot be installed on Windows XP nor on Windows 2003. A temporarily solution can be found here: http://travisspencer.com/blog/2009/02/installing-geneva-framework-on.html
Happy Claiming!