Hi Michael;
until now PB15 is a castle in the air. I would not count on it. So IMHO:
So to my real question. What is a PB web service capable of now and in PB15.
You can create SOAP web service in PB Classic. You can create WCF services with PB.NET like Chris said. But I would NOT recommend it. In my eyes PB .NET is not ready for production and migration is a hard journey with uncertain outcome.
If you want to upgrade your application to a modern HTML5 web application you will have to do a lot of refactoring. I have no idea what's the scale of your application is but I would consider to encapsulate you PB business logic to nonvisual objects, wrap it by a ASP .NET Web API Controller (MVC) application and code the representation layer in JQuery and HTML5. Visual Studio is a nice tool to test, debug and deploy your solution.
Someone will feel that posts like this are not really loyal to the powerbuilder product. But at the end of the day we are all measured by the job we did, not by the technology we used or the loyality we spent.