I've created a Haskell module for interfacing with Bitcoin's JSON RPC. You can find Network.Bitcoin
on Hackage. That page links to the source at GitHub. Hackage hasn't generated the module's documentation yet, but it should appear within the next 24 hours.
Some of the most commonly used RPC methods have convenient wrappers. For all others, you have to use a lower level function. The Hackage page gives some brief sample code. If you want more, just ask.
You can post bugs here or on the GitHub issue tracker.