I'd expect that damage threshold will also apply to melee weapons in some way, however, whether different melee weapons will actually have different armor peircing capabilities or if it's just a generic either it does enough damage to get through the damage threshold or not type thing.
I seem to recall (or think I do), that there will be some sort of minimum damage always done; (I guess you can beat them to death with a stick).
If that were the case, it would make things in practice the same as how damage resistence was handled in Fallout 3, I think, seeing as either way, you would still do a small amount of damage no matter what happens. The way that damage is calculated might be different, but as far as the result players would actually see during gamplay, it would probably be pretty much the same.
But my impression is that with damage threshold, if you're attack isn't strong enough to pierce through your target's armor, you won't do any damage at all, contrary to the way Fallout 3 handled armor where it reduced damage by a certain percentage and thus at maximum damage resistence every attack would still do a small amount of damage.