We're due with our first DC in a couple of days and my DH has decided he wants us to start calling each other 'mother' and 'father' Yes its 95% tongue in cheek but I reckon that last 5% is deadly serious, but then he's always been a bit, um, eccentric - thats why I love him!
I agree with comments on regional variations - for instance a lot of my family are from the County Durham and Teesside area, and it was fairly standard when I was young for kids to use 'mam' as opposed to 'mum', as my dad did to his mother. My step-mum called her parents mummy and daddy though until they passed away when she was in her fifties, which contradicts my previous sentence! I think thats a bit odd, and stopped calling my dad 'daddy' when I was about 14 I think.
Its a lot to do with individuals perception of a 'title'. I use my step-mums forename, but when referring to her I don't use 'step-mam' because I think it sounds a bit rough with my accent (sort of mish mash northern).
My step-mum refuses to be called 'granny' or 'grandma' because she thinks it makes herself old and grey - instead she prefers 'nanna'.
My MIL (from Cheshire) prefers 'grandma' because she thinks 'nanna' sounds common! I used 'nanny' for mine, but I was brought up by her, so it was kind of a substitute for 'mummy' My own mother calls herself 'mom', as she's lived in the USA for nearly 30 years, but I use her forename as I don't know her very well. Otherwise I think its a bit insulting to call a parent by their forename, as if they have been demoted from their important role in some way