Dave, web browser capabilities pragmatically drove JSON's Javascript-like format. Unlike XML-based protocols, you can use a JSON service across different web sites with existing browsers (without a server side proxy, Flash, etc.).

I suppose you could base the protocol on XML if you really wanted to, but you'd still have to provide a Javascript wrapper. Fewer browsers support natively parsing XML than Javascript, and performance would suffer. We do all have Ghz machines nowadays, but today's browsers seem to need that and more; I can't believe Firefox still manages to hang for seconds at a time on my Mac Pro.

Sites often offer JSON APIs in addition to XML APIs to support pure browser-based usages. del.icio.us has had an XML API for a long time; they use RSS! If I were calling an API from Java, I'd use the XML not the JSON API.

That said, I'm personally not a fan of the current JSON spec. If you structure your Javascript like the following example, you can support complex graphs, circular references, etc., whereas the current spec. only supports hierarchies: a={};b={};c='foo';a.foo='c';a.bee=b;b.ay=a;