Interesting thread - I have stumbled about that myself. AFAIK, in Germany you usually call your parents Mum and Dad. The only people I know who don't have logical reason to as they have a stepmother or - father while still in touch with their biological parent, so it would be sort of confusing to refer to both as Mum or Dad.
I call my parents Mum and Dad - always have, always will as anything else would be odd to me; maybe not that much to them though, coming to think of it. At least my Dad would not mind I guess since sometimes I call him by his first name (Alfred) in jest when he has asked a stupid Q or done sth. weird and is making fun of it himself. :p I cannot remember I ever referred to my Mum as Verena, though I could not say why. :scratchhead The idea is just unnatural to me.
I believe (like David) that Johnny calls Patti Mom more often than not when they talk. It must come across a bit childish to say things like "My Mom will deal with that" in official negotiations or similar, so he might say Patti instead of Mom because of that.

Vicky II
*** Keeper of Johnny's Magic ***

The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true.
- James B. Cabell -