One of my favorite BizTalk gurus is Richard Seroter. Richard is one of the those talented, visionary guys that I've always enjoyed working with and around. While he recently left Avanade to join Microsoft, we stay in touch and I read his blog religiously.
He just published a great step-by-step example of Building and Executing a BizTalk Single Sign-On Scenario; I recommend you read it if you see a SSO opportunity in your near future, and bookmarking it you don't (sooner or later you're going to need it to pass a security review - the question is not if, but when). And regardless, I highly recommend subscribing to his blog!