John,  (01)
>Petri nets are a good message-passing model, which subsumes finite
>state machines, flow charts, and many kinds of concurrent processing
>as special cases.  (02) 
I found (http://workflow.healthbase.info/) following argument regarding Petri 
nets quite relevant to this discussion:
"...States are not necessarily related to activities or
events. This means that traditional object life-cycle models are inadequate.
Traditional (Petri-net based) models of object life-cycles model closed systems
and so assume that a state results from a transition."  (03)
I might add that REST-based interaction by definition limits the scope of state 
to single interaction. I can't help but to recall 2000 movie called "Momento" 
in which main personage is afflicted by brain disorder preventing any new 
memory from being formed.  (04)
I believe that in principle stateless protocols are not sufficient to support 
SOA. The alternative approaches are generally known as "distributed shred 
memory" had been researched for a long time, but were not economically feasible 
until now.  (05)
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