Mobile video is becoming extremely popular, and P2P mobile video platforms are being considered for large-scale deployment in this context. However, the design and deployment of realistic P2P video systems have to consider specific characteristics of mobile networks. In this article, we look from the viewpoint of a large-scale commercial P2P mobile video provider system, PPTV, and describe the implementation challenges of a P2P mobile video system over 3G networks. Our analysis is backed by real measurements and experience from PPTV. We extract from these measurements the characteristics of mobile videos and analyze their impact on P2P video systems. We also briefly discuss other practical issues in the design of a mobile P2P system for PPTV.