While reading up before the UU Cheminformatics journal club, initiated thanks to Egon Willighagen, I stumbled on this ... which seems to be something for Bioclipse, whenever it's time for extending it into the systems biology direction:
A LGPL licenced, Java based, Stochastic biological system simulator, designed with ease of integration and interoperability in mind. (Stochastical simulations seems to be the ones which currently captures biological behaviour the best).