This is some great work on federated security in healthcare done by several researchers at the University of Virginia, dept. of Computer Science (full disclosure: Microsoft funded some of the papers).

The "Papers and Presentations" section has a lot great articles, tutorial and demos on the topic of federated security in healthcare using Web Services and the federation standards.

If you have anything to do with Healthcare Information Exchanges across multiple organizations in a highly distributed environment, you should defititely check them out.