I just got a comment from Oran about the lack of durable messaging in WCF and the need for a respective extensibility point. Well... the thing is: Durable messaging is there; use the MSMQ bindings. One of the obvious "problems" with durable messaging that's only based on WS-ReliableMessaging is that that spec (intentionally) does not make any assertions about the behavior of the respective endpoints.
There is no rule saying: "the received message MUST be written do disk". WS-ReliableMessaging is as reliable (and unreliable in case of very long-lasting network failures or an endpoint outright crashing) and plays the same role as TCP. The mapping is actually pretty straightforward like this: WS-Addressing = IP, WS-ReliableMessaging = TCP.
So if you do durable messaging on one end and the other end doesn't do it, the sum of the gained reliability doesn't add up to anything more than it was before. MSMQ is fully in control of both ends of the wire and makes assertions about the endpoint behavior and was therefore the logical choice for our durable messaging strategy in V1, because it already ships with Windows and there is (as of yet) no agreed interoperable set of behavioral assertions for WS-RM around how endpoints must deal with received messages except ACKing them.
See Shy's comments.