I think it's not technologically unimaginable -- we know basically what we'd need in order to get there. With a nuclear pulse design, plus sufficient life extension to make people willing to go on 200-year voyages, no reason why the Gliese 581 system can't be our second home. (Hopefully we'll think of a catchier name by then).

Taking a really long-term view, Gliese 581g has certain advantages over Earth. Its M-class sun will still be burning many many billions of years after our fuel-hungry sun has burned itself out.