JMS defines two different type of information which can be associated with a message (apart from the body): message headers and message properties. Message headers are accessed using specific javabean-like methods such as getJMSPriority whereas message properties are accessed using generic methods such as getIntProperty(name).
JMSXDeliveryCount was defined (though not made mandatory) in JMS 1.1 and is a message property, not a message header, which is why it is accessed using setIntProperty and getIntProperty rather than more specific methods. In JMS 2.0 all we are doing is making this existing property mandatory.
I can see a case for defining specific methods to access the JMS-defined properties (those starting with JMSX), or even just the mandatory one(s), but there is potential confusion in providing two ways to access the same value. And since JMSXDeliveryCount was defined in JMS 1.1 it has to remain a property.
So I propose we leave things as they are. However I'll leave this issue open for now to allow for other comments either via the issue or via the EG/user alias.