I saw this via KurzweilAI.net…

Photovoltaic Moore’s Law Will Make Solar Competitive by 2015 (Tech Talk)
Silicon-based solar technology has decoupled from the semiconductor industry and is achieving steady cost reductions, so that those following PV discern a kind of Moore’s law at work. In 2005, production of silicon for solar cells already surpassed production of silicon for semiconductors.

They make other points that could make it viable in the next 5 – 8 years…Interesting article.