I'm actually surprised at how FEW people on this thread do this. I thought it was much more common.
I call mine mom and dad, no problem. In fact my in-laws also refer to one another as Mom, Dad, even "your mother" when in the presence of their children-in-law only. 4 kids and all the in-law kids call them mom and dad.
DH on the other hand, calls my parents by their first names, as that is what he's most comfortable with. I don't think either set of parents care either way.