CEP platforms, such as StreamInsight, are an enterprise-class solution for processing multiple events at high volume and applying pattern matching algorithms for these events.
In CEP platforms, there are many domain-specific events, such as event classification (point event, interval event, edge event), which are not first-class in Rx.
In short, it looks like you could create a CEP platform using Rx, but it is not useful as a CEP platform in itself. The analogy I would draw is Rx - this is CEP, since WCF belongs to nServiceBus. nServiceBus is a communication platform, but it has much more domain specificity than in WCF (where in this case the domain is an enterprise level messaging).