In truth, the two words don't have the same meaning, even though they are often used interchangeably. Acclimate means to adjust to a single environmental factor, where as acclimatize means to adapt to many. Acclimatize is generally used for adaptation of organisms to changing seasons in the wild, where as acclimate is generally used for changes in a laboratory conditions in which factors are controlled. I have had a professor or two that have been very uppity about the distinction. To tell you the truth, however, I don't know which is more appropriate for adjusting an octopus to a new tank.
Oh, its worse than that... We even spell it differently 'acclimatise' no 'Z'
I think it depends on the context and acclimatise sounds like the act of acclimating... actually, i hate the sound of the word 'acclimate' and always use acclimatise. Its true that no-one over here uses acclimate.
Well, as an aussie who is now in the US, I say acclimate now but definitely acclimatise was more common when I was at Melbourne Uni. It seems to be a reversal of the normal trend in the US to add extra -izes and -ates onto verbs. (My pet loathing is 'burglarized' which news readers say all the time here...its burgled! /end rant/...).
I have always used acclimatise in my vocabulary as a synonym to adapt (regardless of whether it's to a single condition or multiple or changing conditions), acclimate is something that I have never really heard people in NZ use..
I don't think people actually give much thought to the different meanings between acclimatise and acclimate. Just a thought!