Looking at their github repository and searching through the source code, I could not find JSON RPC API, except the one for Whisper. Perhaps I did not look hard enough?
02/04/2016 I had a better look and no, there is no JSON-RPC API service.
07/04/2016 - The new changes to be merged that was referred to by @Oscar Guindzberg at https://github.com/ethereum/ethereumj/pull/352/files has a JSON RPC API with jetty for the HTTP handling and jsonrpc4j for the JSON conversions. Nice.
There is a non-merged pull request you can try https://github.com/ethereum/ethereumj/pull/352