I'll give you a serious answer. There has been a lot of pissing and moaning about people who give smart ass answers, and I've started to lose sleep about it.

arty:
You are using smallish tile. Let's say you are tiling three walls, like a tub surround. Take the 5' wall first. Let's say that both the end walls are out of plumb-they're tilting in so hard that the 5' wall is like a triangle. It starts at 5' wide at the base and ends 8' up at 0 wide.
So what do you do? Just tile it. Lay it out from the center, so that you won't have small tile on the ends. You're probably using a half bond pattern anyway, which will help. Get your tile tight into the corners, say within an 1/8".
Now the side walls. Let's assume that your long wall was wack as well and leaning away (out) at the top. Strike a plumb line somewhere, and lay into the long wall from the plumb line to get your layout going. Cut your tile where they meet the wacky wall so they are angled (scribed) to fit it.
Your corners should have a straight (not plumb) line running up them, not a staircase effect.
That should do it. Hook up the exhaust fan to the plumbing vent line and it's time to move on to the next job.