As it turns out the initial release of MidiSerial had some problems sending messages to the Arduino. It seems the FTDI virtual comm port drivers are a bit particular about the termios settings they expect – at least my simplistic approach to configuring a raw serial connection was not good enough.
Having come across this excellent article on todbot I have now managed to sort the problems out.
Up-to-date source code is available on our git server and github.
You can also download a Mac OS X Universal binary from here: MidiSerial.