Deciding on services with SOA

Something I heard in few SOA lectures I have been to was the question "how do I decide which services are in the system?", to which there was never an easy answer.

Breaking the system into those services is not an exact science, so two architects may come up with a completely different set of services for the same system.

Udi Dahan talked about a simple method in an ARcast he did few months ago, mainly using transactions to define services - since distributed transactions are complex by nature, you should avoid them, so if you need a transaction for a set of actions, you should bundle that set into a service.