We consider the problem of content-based routing and dissemination of highly-distributed, fast data streams from multiple sources to multiple receivers. Our target application domain includes real-time, stream-based monitoring applications and large-scale event dissemination. We introduce SemCast, a new semantic multicast approach that, unlike previous approaches, eliminates the need for content-based forwarding at interior brokers and facilitates fine-grained control over the construction of dissemination overlays. We present the initial design of SemCast and provide an outline of the architectural and algorithmic challenges as well as our initial solutions. Preliminary experimental results show that SemCast can significantly reduce overall bandwidth requirements compared to traditional event-dissemination approaches.