Avnas, splines were originally thin flexible pieces of wood used in construction to help make curves. Math uses the word to describe a particular type of curve and it's used in computer graphics to describe a tool that's used to help produce smooth curves (instead of trying to fake them with polygons). I played the sims a lot, once upon a time and always guessed it was a semi-nonsensical way of saying "we are doing some arcane computer stuff behind the curtain here to get the game ready for you."
I kind of figured it had popped up here as an obscure reference to the sims...
Oh. I guess I'm too geekish for my own good, as I always thought it was a serious processing notice. And no, I just can't stand Sim-anything games, so I never met the original joke in the first place.