The Blue Gene/L supercomputer will use system-on-a-chip integration and a highly scalable cellular architecture to deliver 360 Teraflops of peak computing power.With 65,536 compute nodes, Blue Gene/L represents a new level of scalability for parallel system, with specific challenges in the areas of scalability, maintenance and usability. In this paper we present our vision of a software architecture that faces up to these challenges, and a simulation framework that we have used to experiment with our ideas.