I'm not really very sure exactly where the Underworld references are pointed, and presumably it can't be too crucial either, as 95% of a modern Doctor Who audience couldn't be expected to pick it up. Still, cautionary tales about striving for a lost paradise, and about the true character of the Timelords and their relationship with other species, seem to be in the air. That certainly fits with what we've been led to expect of this season, anyway.