However it seems to me that doing this would be very difficult for at least 2 main reasons.
First spreadsheets can be organized in essentially arbitrary ways and there is no standard way of defining the semantics of data in them. There can also be all sorts of references between data items and unlike SQL there is nothing to explicitly define the nature of these references. How could a program take an arbitrary collection of spreadsheets and transform them into a webapp that has significantly more functionality than just loading them into Google docs ?
Second spreadsheets often use macros and other programming constructs so I think you also have the problem of building a general translator from macros/VB/Excel functions to javascript/whatever backend language you're using. This again seems like a fairly difficult problem to tackle in general.