Regardless of whether you use web services to serve the XML or you use REST services, in either case the success of your project depends on how you design the interfaces to these services.
With the advent of AJAX, asynchronous is all the rage. If you're not familiar with AJAX, if you've used Google Maps you've seen AJAX in action - it's what allows the map to be updated while you are doing other things on the page.
I've heard a number of people recently take this as proof that asynchronous messaging is the "right" way to communicate. Too bad they are comparing apples to oranges.
While that's interesting, the real important topic to discuss is how the use of AJAX changes the way you need to think about designing services...