I have a 430EX bouncing off the ice in front of her, with the diffuser pointed up, and now that you mention it, the ice behind her is in focus... Tried to select a focus point on the 30D... If I remember correctly, I've read somewhere that people had problems with that feature not working quite right?
The main focus of the pictures I shot at the Ice event was playing with my 430EX with such weird lighting. Blue lights, amber lights, red lights... I removed the color from the ice as well. Looked neat.
Except in a rare few instances I always use a single central focus point. The sequence is usually expose and lock, focus and lock, recompose and click. This just sort of carried over from manual FM2n days and then from F4s days.