Just to add to the discussion - I'm Canadian and finished university there, and I NEVER used -ise. Hardly ever saw it either. -ize is definitely the dominant form, used in primary school spelling lessons, newspapers and academic university texts (which are often American of course).

Now I work in Europe and try to use British spellings when writing in English for my company. But what I have noticed is that only the British use these spellings...non-native speakers of English generally use spellings like 'optimize'. I found this page because I'm now considering just sticking to 'American' spellings - I would have a lot fewer corrections to make in the texts I proofread!

Even if the 'standard' form is -ise in several English-speaking countries, I think that -ize can be considered the global norm.