One of the major shortcomings of SOAP 1.1 is that it requires all calls to the API to happen via HTTP POST, which effectively cuts off many standard and emerging technologies such as XSLT, XPointer, XLink, etc. What makes Xoomle interesting is that it opens Google up to these technologies, allowing Google to be combined with other services in interesing ways.
Amazon for example already provides a REST interface to its services. Using a relatively simple XSLT stylesheet, I put together a demo that takes the title of a book, uses Amazon to find the name of the author, and then uses Google (via Xoomle) to provide a link based on the name. Granted, this is rather trivial, but it begins to show how disparate services can be brought together to provide a composite service.
The details are here.