OK - WIF/ADFS is definitely the way to go - but there are some pre-requisites
- for your infrastructure - a Windows 2003 domain is fine, but ADFS needs to be installed on Server 2008 (R2). You also need 2008 licenses.
- for the clients - if your backend would be solely WCF, this would be easy since client support is built into WCF. But more "exotic" things like MSMQ (System.Messaging) or NServiceBus may require you to manually talk to an STS. The classes for
that are in WIF, but XP is not supported in that scenario.
That said - you should definitely use WIF (and the claims based model) on the server side.