Because of the emphasis on creating stateless Web Services state management has become a big issue in SOAs. Many rely on IMDBs, however, you can also utilize the WS-Coordination services to manage context information. The WS-Resource framework has also been recently released, and deals with state management head-on.
WS-Resource is designed to work with IBM's WS-Notification specification. Back when IBM released this standard, it parted ways with Microsoft who came out with a similar specification entitled WS-Eventing. It's hard to say where this is all going, but some analysts are saying that these are simply posturing moves, and that they will eventually merge their efforts into a single standard or framework.
Just a comment on idempotent and stateless. Suppose I write a webservice to return the current time. It is stateless but not idempotent. But if a service return the sum of two number, then it is stateless and idempotent.