Distributed sensor applications emerge as a promising solution to be utilized for complex business scenarios. However, the development and deployment of these applications remains a complex challenge. In this survey we present a two-dimensional classification of middleware technologies needed to realize these complex end-user business scenarios. Subsequently, we discuss currently existing middleware approaches and place them in the classification framework we developed. Finally, based on this classification overview, we identify two issues for which middleware support leaps behind: cross-layer integration and end-to-end integration.