Had mine done, around 80k. Advised that the you only change the water pump if it starts to fail.
DON'T RISK IT! Had a water pump fail on my Calibra Ecotec......£1500 to fix it.
Water pumps are around £45 - 60, for the little extra you have piece of mind. Plus it makes sense, that if the cambelt is off, to change the water pump as there rum off the same belt.
Make sure you tensioners and pulleys are changed too.