It all depends how clever the EPG is with Sky HD....
I suspect it will be as crappy as the current one.
Thus you will be able to use Sky HD to watch / record the few HD programs / movies you like while Tivo records everything else.
Many Tivo users have also made the "Mode 0" mode to increase the picture quality of Tivo to near DVD.
Only downside is the extra monthly runnings cost.
Automan.