Piggieback doesn't seems to work when you start the nREPL server manually, as stated in the documentation.
To check see do this:
lein run
# from another console:
lein repl :connect 5050
(piggie-doesnt-work.core/start-browser-listener-from-Nrepl)
Now a long "java.lang.RuntimeException: No Context associated with current Thread" traceback will be displayed.
Starting the nREPL using "lein repl" seems to work ok.
Lein version: Leiningen 2.0.0-preview10 on Java 1.7.0_05 Java HotSpot(TM) 64-Bit Server VM