Most of the answers out there are wrong. If it's a unibody Macbook Pro, the power button is built into the keyboard.
Hence, if you kill the keyboard, it might not turn on. But the logic board might still be fine. There are two tiny holes liquid can seep into.
If it makes it onto the logic board, it's got to be a sizable spill. The The liquid goes through the keys, the backlight, and then the black cover. (All of which brilliantly have a hole cut into it)
If you spill on the right side of the keyboard, the water hits the DVD ROM. If the water then rolls left, it hits most of the main components at the connection level. Probably the worst case scenario.
If you spill on the right side, the water hits the RAM and a whole sea of capacitors there. Might be recoverable if you shut it off quick enough.
Whatever you do, turn the thing upside down immediately. Unplug it, open the back of the case and pull out the cable that connects the board to the battery.