Does anyone know if the T7 phage promoter is efficient in mammalian systems?
See High level gene expression in mammalian cells by a nuclear T7-phage RNA polymerase. The authors recognize that the T7 RNA polymerase tends to work only in the cytoplasm thus is unable to transcribe genes in the nucleus. To remedy this limitation, the T7 polymerase was fused to a nuclear location signal peptide to localize the polymerase correctly.
Promega sells a T7 expression system for RNAi production.