BD Biosciences Clontech has recently introduced the BD Knockout inducible RNAi system.
According to the company, the system features inducible retroviral shRNA expression vectors, which are provided linearized and ready for ligation with a dsDNA oligonucleotide encoding an shRNA. It uses a modified tetracycline-controlled gene-expression system in which expression of an shRNA is induced when either tetracycline or doxycycline is added to the culture medium of an appropriately engineered cell.
Additional details about the inducible RNAi system can be found at http://www.bdbiosciences.com/clontech/archive/JAN05UPD/inducible_rnai.shtml.