And equally impossible to do in a way that is globally useful.
If a very particular industry willing to open up their data in standard and extensible format (may be RDF or a simpler standard), like film theater listing their showtimes or governments release their statistical data in RDF rather than Excel only, many useful application could be developed on it and it is a big step towards semantic web already. And I think it is not too hard to do.
It's complicated as hell to do simple things in RDF, but that's a different issue, not the one I struggle with every day.