Seems they alluded to exactly that some years ago (2008!): http://www.teslamotors.com/blog/mythbusters-part-3-recycling... "... at some unfortunate point, the ESS will come to the end of its useful life in the application :(. However, it might be possible to use the ESS in other applications. For example, the ESS could be used as a power source for off-grid backup or load leveling. "

I think they've been doing it in one-off situations with Solar City now for a while. If they are not doing exactly this with this venture, I'd be surprised.