*sighs* That's the wrong blasting. XML and YAML don't solve the same problem. XML solves the problem of how to encode arbitrary data + metadata into a text format. YAML solves the problem of how to encode arbitrarily-nested data structures. AFAIK, YAML doesn't address the issue of metadata.

Now, YAML should be used in many places that XML is used. But, if you had to choose only one to use for every single arbitrary data (+metadata) need, that solution would be XML. I'd love to be proven wrong re: YAML's inability to handle arbitrary metadata, but I don't think I am.