The explanation I heard for US/UK spelling differences (I have no idea where I heard it -- therefore no idea if it's true or just urban myth; but it makes sense) has to do with dictionaries. For a long time, there was no 'standard' spelling -- everybody spelled how they wanted to. I remember reading stuff in classes where one author would change their own spelling!
Then came the dictionary. With 'official' spellings and definitions. Dictionaries were being created in the US and the UK around the same time -- and that's when the US chose a different spelling. Whether it was, as LabRat suggested, to make things simpler for immigrants or, as I had heard, sheer obstinancy and desire to further enforce 'independence,' I have no idea. My gut says sheer obstinancy -- if we truly had been trying to make things simpler for immigrants, there are way too many words we missed!
Bethy
