Comment of the Day: The Dreamwork of Humanity: "Re: 'The Minds have run a billion simulations...:
...of every course of action (and are the programs inside their simulations self-aware? Horrible thought!)"
This is mentioned in Hydrogen Sonata, the last Culture book, IIRC. There apparently are genuine ethical issues with sufficiently accurate simulations, with approaches varying: some civilizations constrain their sims to the levels where the simulated entities don't become self-aware; others set up virtual worlds for their sim-people to live in after they've been useful; and yet others just terminate the simulations with glee.
As for the white winged unicorn, I'm pretty sure you're thinking of Princess Celestia. Praise the Sun!