Wireless sensor networks research is primarily focused on various internal sensor network issues like routing, self-organization, MAC layer design, data aggregation, etc. Interaction between sensor networks and especially such interaction over mobile networks has not been researched well. In this paper, concept of mobile peer-to-peer sensor networks, i.e. peer-to-peer sensor networks overlay on 3G mobile networks is presented. Each sensor network acts as one peer node and is represented by its gateway in a P2P network. Peers use mobile networks to communicate and collaborate on execution of specific tasks or to provide information to users that otherwise would not be available. Usability of JXTA as P2P framework in mobile environment has been investigated. Architecture of a sensor network gateway is also presented.