Anyway, that alone doesn't exactly constitute a reference - it's that he gives you a quest called Little Sisters (the Little Sisters are a major plot element of the BioShock games). How could that possibly be a coincidence?
Just raised my eyebrow. That kind of subtle acknowledgment is usually reserved for universal classics (e.g. Mario, Final Fantasy) or in-company properties (try to find a Namco game that doesn't reference Pac-Man). Since there is no developer/publisher/staff link between The Witcher 2 and BioShock, it seems to imply the former case, which would suggest BioShock is being canonized.
Or, much more likely, that some bored voice actor was having himself a little joke when he saw the name of the quest he had to read.