Today we lack an infrastructure for globally processing stream data from sensor networks and making this data available to millions of users in real-time. To build such a system, we need to address a set of challenges and, in particular, rethink what dependability means in this context: it is infeasible to guarantee perfect data processing at a global scale. Instead, the degradation of result quality due to failure and resource shortages should be made explicit to users. We briefly describe one such model to achieve a dependable Internet-scale stream processing service.