Microsoft is providing a CTP of their Internet Service Bus (ISB), which will allow developers to easily build publish/subscribe technology based applications. The Microsoft ISB is currently being called BizTalk Services and you can take a better look at http://www.biztalk.net. According to John Shewchuck, in a very informative video at Channel 9, the name comes from the classic BizTalk server being an Enterprise Service Bus (ESB) and it made sense to brand this new Internet Service Bus under the same brand, that is BizTalk.
Publicly well-known publish/subscribe based applications are chat rooms, MSN and Skype. A more business-oriented application might for example be a business document sharing application that would allow one company to access their business documents from another company.
The BizTalk Services comes with an SDK that allows developers to create a conventional WCF service that can be published on the ISB and then build a client that connects to this service through the ISB. But for further information I would suggest that you first take a look at the video and then try it using the SDK.
Anyway, I think this technology is really cool and will open up a new way of publishing services based on the NET·Conductor technology from Rue de Net that I'm involved in developing. The NET·Conductor allows web service based access to data and logic within Microsoft Dynamics NAV business applications.