Two different programs that use the same XML Schema - that write to the same interface specification. It's just an API. No, not magic pixie dust, but it helps. I think we'll get standard XML Schemas (like the per industry efforts), for specific purposes. It's not really the XML that helps, but the standardization of the interface. But even that's hard. However, there's a lot of money to be saved and agility to be gained, so this (or something like it) is inevitable. It's a worthwhile endeavour.

I was mainly addressing your comment elsewhere about people who like to lock things down, and specify them upfront. For interfaces, you really do need to agree on some things, and be strict (within those tolerances). Someone changing an interface on you can be pretty frustrating.