Michael Platt comes up with a nice distinction of interoperability vs. integration:
Interoperation is a peer to peer sort of thing where the two systems interoperate between themselves sharing information (which may be state and so can change processes in one another). Interoperation is more allied to a web services set of concepts and SOA.
Integration is where a system is subsumed within another either conceptually (one black box holding and hiding another) or physically (a web front end added to a mainframe application). ESB’s and Enterprise portals are more akin to integration systems.
Fits my SOA world view very nicely.