There's a new article in the developerWorks WebSphere zone, "Introducing XMS -- The IBM Message Service API." XMS brings a JMS-like API to non-Java clients.
XMS, the IBM Message Service API, provides a consistent messaging API across client languages and across messaging products. The messaging products currently supported are WebSphere MQ (duh!), WBI Message Broker, and Default Messaging (part of the SIB) in WAS 6. The client languages supported are C/C++ (see IA94: IBM Message Service Client for C/C++) and .NET (see IBM Message Service Client for .NET Beta). So if you're using the XMS API, even as you switch languages or messaging providers, your API stays the same, helping to minimize code changes. Much like the way JMS enables Java/J2EE programs to more easily switch messaging providers.
So far, XMS is just an IBM thing. However, I think IBM hopes that other vendors will adopt it for their messaging products. Then customers will be able to develop code that is more product- and vendor-independent.
The WebSphere MQ API (called MQI) also supports C/C++ and C#/.NET clients (as well as a slew of other languages). But XMS has some advantages: The pub/sub support in XMS is better, because you have methods to subscribe and unsubscribe, whereas with MQI you have to send those requests as messages. Meanwhile, Message Broker and Default Messaging don't really support C/C++ and C#/.NET at all, but now they do via XMS, and in a way such that the client doesn't really care which provider it's using.
So if you're using C++ or .NET and want to use the IBM messaging products, check out XMS, it'll make your job easier.