I can't wait. I've been working with a bunch of XML files (TEI corpora, actually), and using document stores to build out indexes. One of the things that I've found is that what I want to do with reduce functions (build complex dictionaries, for instance with the key being a word stem and value being another dict with key=word form and value=list of locations or related words or whatever) is not the kind of thing Mongo/Couch DB reduce functions are designed for (i.e. scalars).
I learned what a 'reduce' is from learning Clojure (which was my intro to functional programming), not from databases. And so I got really used to the idea of reducing into nested dictionaries, because Clojure is good at that. Which meant that when I came to using MapReduce in DB environments (particularly CouchDB, but also a lil' Hadoop), I was a little disappointed.
Which is to say that a solid Clojuric abstraction layer that emphasizes a bucket approach on top of a variety of different stores sounds like my dream library.
