how do you spell 'mom', mum?

I always have a problem with American spelling versus the correct spelling of certain words when it comes to the world wide nature of the Interweb. How am I ever going to buy a t-shirt with the words 'tyres,' 'organise,' 'theatre,' or 'centre' anywhere on it? What if I find a great tee which says, 'organize the tires into the center of the theater'? Should stores offer alternate spelling versions of their tees?
This tee is a perfect example. Can I in all honesty wear a tee which boldly flaunts the American spelling of 'mum'? Thinking about it, I guess this is the most logical spelling of mum, as it comes from the word 'mother'... well I guess it could come from a latin word like 'mumtabular' meaning 'mother', and if that's the case then 'mom' isn't correct. Good thing that's not the case.

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For: US$22.95