People are doing small molecule screens with stem cells right now (albeit embryonic stem cells). Probably a lot more screens of that kind are in progress behind closed doors in the pharmaceutical industry, and we can expect iPSC culturing to get better and cheaper, so sometime in the next decade we'll see them become a common model.
iPSCs will probably be an improvement on existing cell culture models, and will also allow things like the creation of cell lines for specific genetic diseases, but they wont' be a quantum leap forward for drug discovery in general.