I have long been a fan of JSON. At RideCharge we still work with xml and soap, and they are fine (still better than FTP drops). The truth is when you have to do B2B work you usually agree to almost anything to get your work done.
But when you release a web service for public consumption, you really need to think about in what format you want to expose your data. We expose most of our APIs in JSON, XML, and YAML. We only get this luxury because it takes almost no effort to do all three. I mean, come on, who would expose something in YAML otherwise J.
That is why it was semi surprising when Google quietly announced their new server-side AJAX API would be JSON only. At first it made very little sense to me, but after some thought, it makes reasonable sense. I declared that 2007 was going to be the year of JavaScript Only APIs (I am a tech blogger not a futurist), because Google released their AJAX Search API as JavaScript only. But now I see it is much more about supporting only one output format, JSON. Whether it is consumed by you, me, Google or Google AJAX JS API. They realize their data is valuable enough, if you want it, you will write:
JSON.parse(response.body)
I think anything but xml (ABX) is becoming the standard.