If you like this, check out http://www.happstack.com/docs/crashcourse/AcidState.html . It allows for arbitrary Haskell data structures to safely and automatically be marshaled in and out of the data store.
Another feature it lacked is replication which I believe was on the roadmap so I'm not sure about its current state.
If you can live with the fact that your application is the only entry point for your data, definitely look into AcidState.
Sorry for the double post, but I couldn't edit my previous reply anymore. You do have a point about SQL being more amenable to outside queries and reports, especially ad-hoc ones. However, take a look at Data.Acid.Remote (http://hackage.haskell.org/packages/archive/acid-state/0.8.2...) which allows you to connect to an acid store in your running application remotely: however, at this time you will have to provide your own security solution for that. Also, for ad-hoc queries to your data you can, probably, bolt on a dynamic Haskell evaluator/interpreter to your application: see http://www.haskell.org/haskellwiki/Safely_running_untrusted_... for a possible solution. That is, of course, not safe (as in Haskell-safe, see the section about exploits), but neither are ad-hoc SQL queries on your live data store.