In Lisp, code is data and data is code ;P so he's designing it in a markup language that happens to be Clojure :D

Almost nobody mixes lisp (or derivatives of lisp) and XML - sexpressions are the typical representation of lisp code and are isomorphic to xml.

Markup only solves the shallow part of the problem anyway. Sure it can help communicate with graphic designers, but it doesn't do everything.

Lots of OSX apps (the ones which most people think of as being friendly, usable and pretty) are done mostly programmatically, even with the interface builder stuff available...

You didn't explain why it's a good thing.

From what I understand, it only complicates development even more without providing any real improvement.

It can be a good thing the way HTML-CSS-Javascript separation can be a good thing. In the case of Clojure, it actually is possible to write the layout code in a declarative style and keep the handlers completely separate, but for most of the popular languages, it's easier to just use Qt Designer, Glade or similar to generate an XML layout and write the handlers in your language of choice. Compared to the old approach of GUI designers generating a pile of C/C++ code, which you then modify, the XML blob is actually much nicer.

Also, most of the XML-presentation libraries offer multiple language bindings, so there's less of a requirement for your language to provide its own GUI toolkit, and it's easier to convert your app from one language to another (e.g. injecting Python into a legacy C app) without losing any of the details from the original interface.

As soon as I can find a well-tested, mature framework for this that has an awesome GUI designer, I'll switch! Until then Swing Designer is my friend.